Read the training manual:

Rat Grimace Scale (RGS): The Manual

Read the original paper:

Sotocinal SG, Sorge RE, Zaloum A, Tuttle AH, Martin LJ, Wieskopf JS, Mapplebeck JCS, Wei P, Zhan S, Zhang S, McDougall JJ, King OD, Mogil JS. 2011. The Rat Grimace Scale: a partially automated method for quantifying pain in the laboratory rat via facial expressions. Molecular Pain 7: 55. doi:10.1186/1744-8069-7-55 

Read papers that validate or use this technique:

Chi H, Kawano T, Tamura T, Takahashi Y, et al. 2013. Postoperative pain impairs subsequent performance on a spatial memory task via effects on N-methyl-D-aspartate receptor in aged rats. Life Sciences 93(25-26): 986-993.  doi:10.1016/j.lfs.2013.10.028

Liao L, Long H, Zhang L, Chen H, et al. 2014. Evaluation of pain in rats through facial expression following experimental tooth movement. European Journal of Oral Sciences 122(2): 121-124. doi:10.1111/eos.12110

Kawano T, Takahashi T, Iwata H, Morikawa A, et al. 2014. Effects of ketoprofen for prevention of postoperative cognitive dysfunction in aged rats. Journal of Anaesthesia 28(6): 932-936. doi:10.1007/s00540-014-1821-y

Oliver V, DeRantere D, Ritchie R, Chisholm J, et al. 2014. Psychometric assessment of the Rat Grimace Scale and development of an analgesic intervention score. PLOS ONE 16(9): e97882. doi:10.1371/journal.pone.0097882 

Préfontaine L, Hélie P, Vachon P.  2015. Postoperative pain in Sprague Dawley rats after liver biopsy by laparotomy versus laparoscopy. Lab Animal 44(5): 174-178. doi:10.1038/laban.731 

De Rantere D, Schuster CJ, Reimer JN, Pang DS. 2015. The relationship between the Rat Grimace Scale and mechanical hypersensitivity testing in three experimental pain models. European Journal of Pain, Jul 1 doi: 10.1002/ejp.742

Whittaker AL, Leach MC, Preston FL, Lymn KA, et al. 2015. Effects of acute chemotherapy-induced mucositis on spontaneous behaviour and the grimace scale in laboratory rats. Laboratory Animals, Jul 10, pii:0023677215595554

Waite ME, Tomokovich A, Quinn TL, Schumann AP, et al. 2015. Efficacy of common analgesics for postsurgical pain in rats. Journal of the American Association for Laboratory Animal Science 54(4): 420-425. PMID:26224443

Gao M, Long H, Ma W, Liao L, et al. 2015. The role of periodontal ASIC3 in orofacial pain induced by experimental tooth movement in rats. European Journal of Orthodontics, Dec 15. pii: cjv082. doi:http://dx.doi.org/10.1093/ejo/cjv082

Yousef MA, Dionigi P, Marconi S, Calligario A et al. 2015. Successful reconstruction of nerve defects distraction neurogenesis with a new experimental device. Basic and Clinical Neuroscience 6(4): 253-264. PMCID: PMC4668872

Asgar J, Zhang Y, Saloman JL, Wang S et al. 2015. The role of TRPA1 in muscle pain and mechanical hypersensitivity under inflammation conditions in rats. Neuroscience 310: 206-215. doi:10.1016/j.neuroscience.2015.09.042

Long H, Liao L, Gao M, Ma W et al. 2015. Periodontal CGRP contributes to orofacial pain following experimental tooth movement in rats. Neuropeptides 52: 31-37. doi:10.1016/j.npep.2015.06.006

Kawano T, Equchi S, Iwata H, Yamanaka D, et al. 2016. Effects and underlying mechanisms of endotoxemia on post-incisional pain in rats. Life Sciences 148: 145-153. doi:10.1016/j.lfs.2016.01.046

Leung V, Zhang E, Pang DSJ. 2016. Real-time application of the Rat Grimace Scale as a welfare refinement in laboratory rats. Scientific Reports 6:31667. doi:10.1038/srep31667

Miller AL, Golledge HDR, Leach MC. 2016. The influence of isoflurane anaesthesia on the Rat Grimace Scale. PLOS ONE 11(11): e0166652. doi.org/10.1371/journal.pone.0166652

Finlayson, K., Lampe, J.F., Hintze, S., Würbel, H. and Melotti, L., 2016. Facial indicators of positive emotions in rats. PloS one, 11(11), p.e0166446.doi: 10.1371/journal.pone.0166446

Saine, L., Hélie, P. and Vachon, P., 2016. Effects of fentanyl on pain and motor behaviors following a collagenase-induced intracerebral hemorrhage in rats. Journal of Pain Research, 9, p.1039. doi: 10.2147/JPR.S121415

Schneider, L.E., Henley, K.Y., Turner, O.A., Pat, B.M., Niedzielko, T.L. and Floyd, C.L., 2016. Application of the rat grimace scale as a marker of supraspinal pain sensation after cervical spinal cord injury. Journal of Neurotrauma, (ja). doi: 10.1089/neu.2016.4665

Long, H., Liao, L., Zhou, Y., Shan, D., Gao, M., Huang, R., Yang, X. and Lai, W., 2017. A novel technique of delivering viral vectors to trigeminal ganglia in rats. European Journal of Oral Sciences.doi: 10.1111/eos.12326  

Philips, b.h., weisshaar, c.l. and winkelstein, b.a., 2017. Use of the rat grimace scale to evaluate neuropathic pain in a model of cervical radiculopathy. Comparative medicine, 67(1), pp.34-42. doi: 1484259021557 

Jeger, V., Arrigo, M., Hildenbrand, F.F., Müller, D., Jirkof, P., Hauffe, T., Seifert, B., Arras, M., Spahn, D.R., Bettex, D. and Rudiger, A., 2017. Improving animal welfare using continuous nalbuphine infusion in a long-term rat model of sepsis. Intensive Care Medicine Experimental, 5(1), p.23. doi: 10.1186/s40635-017-0137-2

Yamanaka, D., Kawano, T., Nishigaki, A., Aoyama, B., Tateiwa, H., Shigematsu-Locatelli, M., Locatelli, F.M. and Yokoyama, M., 2017. The preventive effects of dexmedetomidine on endotoxin-induced exacerbated post-incisional pain in rats. Journal of Anesthesia, pp.1-8. doi:10.1007/s00540-017-2374-7

Gao, Z., Cui, F., Cao, X., Wang, D., Li, X. and Li, T., 2017. Local infiltration of the surgical wounds with levobupivacaine, dexibuprofen, and norepinephrine to reduce postoperative pain: A randomized, vehicle–controlled, and preclinical study. Biomedicine & Pharmacotherapy, 92, pp.459-467. doi: 10.1016/j.biopha.2017.05.038