The dose-response of serum prolactin (PRL) values to different doses of naloxone (NAL) in rats is bellshaped. To test the effect of different doses of NAL on serum FSH, LH and PRL in humans these hormones were measured in 4 male volunteers after subject blind infusion of 0.02, 0.2 and 0.5 mg/kg NAL in random order. A saline infusion served as control. On each occasion an adjusted volume of 100 ml was infused within 10 min via an antecubital vein. Consequently blood samples were taken every 15 min for 4 h. To compare the response to the different treatments for each hormone the average of an individual's hormone values (4 x 16) was set to 100%. Values at each timepoint were expressed in relation to the volunteer's mean. The means and SE of hormone values set to 100% were: LH = 5.7 +/- 0.7 ng/ml; FSH = 3.1 +/- 1.0 ng/ml and PRL = 186 +/- 37 muU/ml. NAL infusion did not affect FSH levels at any dose level. No overall effect of NAL on PRL levels was detectable. There was a significant difference in LH values between saline control and NAL infusions, although no significant difference could be detected between different doses of NAL. Higher doses of NAL merely sustained LH levels for a longer time period. Based on the data presented a short test is suggested for screening patients under different pathophysiological conditions for involvement of endogenous opioids in the regulation of LH secretion.