In a controlled, randomised, prospective, clinical study, the effect of prolactin suppression and clinical course of the lactation suppressors Bromocriptine and Metergoline were investigated. During 7 months 150 patients were studied. 81 of those patients, who did not nurse, were treated by Bromocriptine (primary lactation suppression: n = 62, secondary suppression: n = 19) and 69 of the patients were treated by Metergoline (primary suppression: n = 54, secondary suppression: n = 15). The drugs were administrated orally to all subjects, dosed 2 x 2.5 mg/d of Bromocriptine for 14 days and 3 x 4 mg/d of Metergoline for 10 days, starting in average after 13 hours. Puerperal suppression of prolactine were compared with randomised breast feeding subjects (n = 30). In Bromocriptine treated women the average plasma prolactin level decreased from 78.4 +/- 22 ng/ml to 17.0 +/- 3.3 ng/ml during five days of treatment. In Metergoline treated women the plasma prolactin level decreased from 129.7 +/- 15.1 ng/ml to 56.9 +/- 10.0 ng/ml during the first days of treatment. Prolactin level of breast feeding subjects decreased from 233.6 +/- 21.4 ng/ml to 185.8 +/- 23.7 ng/ml during the same period (p < 0.05). There is no statistical significancy of clinical difference of both drugs, but a statistical trend was seen. With Bromocriptine treated women were suppressed efficiently in 71 of 81 cases, 10 refused. Refusals were divided in two quality levels, level I with subjects with moderate complaints and little puerperal lactation, level II with subjects with considerable complaints including strong puerperal lactation. With Metergoline suppressed women, treatment was efficiently in 51 of 69 cases, but refusals of level I were observed in 11 cases and refusals of level II were observed in 7 cases. The results show that Bromocriptine and Metergoline are effective on suppression of lactation. Under the current drug dose of Metergoline an advantage of Bromocriptine were observed. Only further studies could investigate, whether an adaptation of drug dose would improve the clinical efficiency of Metergoline.