OBJECTIVE Thiols (sulfydryl compounds) have a prolactin-lowering effect in animals. The purpose of our study was to evaluate the effects of orally administered N-2-mercaptopropionyl-glycine (tiopronin) on suppression of lactation and plasma prolactin (PRL) levels in a group of women in the puerperium. METHODS One hundred women aged 17-37 years were studied. We divided our population in four groups (25 patients in each group). In group I, no medication was administered but only conservative measures were taken (restriction of fluid intake, ice-bags applied on the breasts, tight breast binders). Subjects in group II received tiopronin (200 mg/day) for 14 days. In group III, tiopronin (500 mg/day) was administered for 14 days. Subjects in group IV, took bromocriptine (5 mg/day) for 14 days. Assessment of suppression of lactation was performed clinically and laboratory determinations were obtained before and after 1, 2, 3, 4, and 14 days of treatment. Furthermore, in seven healthy women plasma PRL responses to thyrotropin-releasing hormone (TRH) before and after one week of tiopronin therapy (500 mg/day) were studied. RESULTS Statistically significant higher percentages of success of lactation suppression were obtained with tiopronin (84% and 88%) and bromocriptine (96%) compared to conservative treatment (60%). In the four groups a significant decrease of PRL was observed. However, in women who received tiopronin and bromocriptine, the levels of PRL after 14 days of therapy were lower compared to the conservative treatment group. Moreover, in the tiopronin groups and the bromocriptine group, lactation was stopped earlier. In the first group (conservative measures) the lactation was suppressed after 13.3+/-5.4 days of treatment. In the groups who received tiopronin (200 mg/day and 500 mg/ day) lactation was suppressed after 4.4+/-1.7 and 4.3+/-1.6 days of treatment. In the bromocriptine group the lactation was stopped after 1.2+/-0.4 days. Bromocriptine treatment was associated with more frequent side effects than the tiopronin. CONCLUSIONS Administration of tiopronin in low to moderate doses is effective in suppression of puerperal lactation.