The day-to-day stability of GABA is an important assumption for many research projects, particularly those investigating neurochemical change/difference as a result of intervention or disorder. While reliability studies have typically found baseline MRS measures of GABA to be stable, some work has pointed to systematic changes linked to the menstrual cycle. Consequently, some researchers have since chosen to include only males within their samples, which can be deeply problematic for the generalisability of findings. Previous work exploring GABA change across the menstrual cycle has involved relatively small samples and been restricted to a small number of anatomical regions. Therefore, we are proposing to extend and expand upon this important work by running a large multisite project using a standardized methodological approach. Our proposal is for each site to collect data from 1-3 naturally cycling female participants before pooling the data. We are very keen to hear from research teams interested in getting involved in the project. Please contact – Katherine.dyke@nottingham.ac.uk for details.