In order to clarify the possible physiological role of endogenous opioid peptides (EOP), we studied the effect of low doses of naloxone (specific opiate antagonist) on plasma prolactin levels in male rabbits. Five groups of five male rabbits each was injected daily between 8-9 a.m., with naloxone 12.5, 25, 50, 100 and 200 microg/kg/day. An additional group of ten animals was injected with saline solution and considered the control group. Blood samples were taken at baseline before naloxone administration and later at 90 min and 1, 2, 4, 7, 10 days after its administration. Samples were also taken 4 days after stopping naloxone administration (day 14). Plasma prolactin levels were measured by RIA. A significant constant decrease in PRL levels was seen with the 12.5 microg at 90 minutes, while with the remaining doses a progressive decrease was recorded throughout the study.