The plasma concentrations of prolactin and nortriptyline were determined in 17 patients with endogenous depression treated for 4 weeks with nortriptyline once a day. None of the patients developed galactorrhoea. A moderate but statistically significant increase (about 25%) in the plasma prolactin was found after 3 weeks of treatment. There was no correlation between the plasma levels of prolactin and of nortriptyline. The results indicate that nortriptyline does not belong to the group of psychotropics which produce considerable increases in the plasma concentration of prolactin.