Grimace scales


Effective alleviation of pain in laboratory animals depends on the ability to recognise pain and assess its severity. Traditional methods of pain assessment based on monitoring of behaviour and clinical signs (e.g. weight loss) are time consuming and can have other limitations (e.g. the signs observed may not be specific to pain).

Research by Dr Jeffrey Mogil and colleagues, McGill University, has demonstrated that changes in facial expression provide a reliable and rapid means of assessing pain in mice and rats. ‘Grimace scales’ have been developed for these species, based on changes in a number of ‘facial action units’, such as narrowing of the eyes (orbital tightening) or changes in the position and shape of the whiskers.

With funding from the NC3Rs, Dr Matthew Leach, Newcastle University, has demonstrated that these facial action units increase in intensity in response to post-procedural pain, and could therefore be used as part of a clinical assessment. He has also developed a rabbit grimace scale.

Where grimace scales are being used to assess pain in real time at the cage/pen side, each animal should be observed for a short period of time to avoid scoring brief changes in facial expression that are unrelated to the animal’s welfare. They should only be used with awake animals. 

VIDEO: Dr Leach discussing methods of pain assessment in animals



The NC3Rs has produced A3-sized posters of the mouse, rabbit and rat grimace scales for display in laboratory animal facility rooms and corridors, to help raise awareness about the scales and familiarise staff with the specific facial action units. These posters are available in English and French.

Due to the COVID-19 lockdown, we are currently unable to send out hard copy resources. Please visit our dedicated COVID-19 page for more information about the support we are offering during this time.


Additional resources

See the links below for guidance on how to use the grimace scales and research papers that underpin and validate this technique.