The effective recognition and alleviation of pain is a critical aspect of promoting animal welfare. In humans and several other mammals, systematic analysis of facial behaviour is effectively used by observers to identify pain states. This methodology is yet to be applied to non-human primates despite their close phylogenetic relationship with humans and their ubiquitous use in biomedical science. We utilised the Macaque Facial Action Coding System (MaqFACS) with rhesus macaques (Macaca mulatta) to determine whether facial muscle movements differ between pain and non-pain states in this species. Opportunistic video footage was collected from experimental animals undergoing potentially painful procedures as part of on-going but unrelated research. Facial movements were coded and compared across four conditions for each individual (n=33): pre-procedure, post-procedure, pre-analgesia, and post-analgesia. Pain was expected to be absent in the pre-procedure condition, highest in the pre-analgesia condition, and present but controlled in the post-procedure and post-analgesia conditions. Facial muscle movements and behavioural dimensions that correlate with pain states, and possibly arousal, are identified and described. We discuss the implications of our findings for the management of laboratory-housed primates undergoing potentially painful experimentation.