To investigate the effect of orally administered testosterone undercanoate (TU) on circulating prolactin (PRL) and PRL response to TRH stimulation, 8 eugonadal male volunteers, aged 19--30, presenting with normal plasma levels of FSH, LH, testosterone (T), estradiol (E2) and PRL, were given 120 mg/day of TU for 6 days. Plasma PRL levels were measured daily during the pre-treatment phase (3 days), treatment phase (6 days) and post-treatment phase (3 days) by radioimmunoassay. The TRH Test (200 micrograms iv) was done on the 3rd day of the pretreatment phase and on the treatment phase. No significant changes in circulating PRL levels or in PRL response to TRH stimulation were observed. Plasma T and E2 levels showed a slight, but not significant tendency to increase, while gonadotropin levels remained unchanged.