To probe the possible involvement of endogenous opioid peptides (EOPs) in progesterone (PG) antagonism on oestradiol-17 beta-(OE) induced uterine cell proliferation, the opioid antagonist naltrexone hydrochloride (NTX) and anti-[Met5]-enkephalin antiserum (AME) were investigated for their effect on uterine DNA synthesis in ovariectomized rats pretreated with OE and PG 24 h before killing. As an index of DNA synthesis the rate of in vitro incorporation of [3H]thymidine ([3H]TdR) into DNA was measured. The inhibitory effect of PG on OE-induced DNA synthesis could be diminished by approximately 42 and approximately 20% by the NTX treatments given directly into the uterine horns 13 and 4h before killing, respectively. Intraluminal AME treatments were only effective when they were administered 13 h before decapitation. Systemic blockade of opioid receptors by intraperitoneal NTX injections given every 6 h to the OE + PG-treated rats did not result in the disinhibition of uterine [3H]TdR incorporation. Our results suggest the involvement of EOPs--including [Met5]-enkephalin--as autocrine/paracrine factors in the PG antagonism on OE-induced uterine DNA synthesis.