Despite abundant evidence of sex differences in the effects of drugs on nonsexual behaviour in rats and mice, most researchers continue to investigate male animals exclusively. This was evident from a survey of all relevant research reports published during the period February 2005-September 2006 (inclusive) in recent issues of five representative behavioural pharmacological journals. Reasons for excluding female animals from most studies are discussed along with attempts to justify the use of either male or female animals only, and the value of including both sexes (especially when a drug effect is poorly understood). Although there are other factors that can influence the effects of drugs, such as strain, age and social density, the sex of experimental animals is the easiest to control and thus is well suited to inclusion in pharmacological investigations. It is accordingly suggested that, as has been recommended many times in the past, animals' sex should play a more important part in future research than is still currently the case.