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Since 2016, the US National Institutes of Health (NIH) has required those it funds to consider sex as a biological variable in their experimental design, analyses and reporting of preclinical studies: in other words, they should include female animals equitably, where necessary for rigour. To explore how the policy has been working, we surveyed the scientists who review NIH grant applications — called study-section members — in September 2016 and October 2017. In their responses, we found cause for both commendation and concern.