Several lines of evidence have implicated the endogenous opioid peptides in the regulation of masculine sexual behaviour. However, although the opioid related peptides alpha-melanocyte stimulating hormone (alpha-MSH) and adrenocorticotropin (ACTH) have been shown to affect lordosis behaviour in the female rat, there is as yet no evidence for a role of the endogenous opiates in the regulation of female sexual behaviour. We present here evidence that the endogenous opiates in the mesencephalic central grey (MCG) are involved in the control of lordosis behaviour in the female rat.