Inhibition of lactation was studied in 38 puerperal women in a double blind trial to assess the effect of bromocriptine in comparison with diethylstilboestrol (DS). Simultaneously the influence of both compounds on blood clotting was studied, along with a control group of 20 women not receiving any medication. Bromocriptine was given in a daily dose of 5 mg for 14 days and DS in a daily dose of 20 mg for 7 days followed by a placebo for a further 7 days. The first doses were not later than 8 hours after delivery. Both compounds showed an inhibitory effect on the onset of lactation and mammary congestion. This inhibitory effect on both parameters was significantly in favour of bromocriptine during the last days of the treatment due to rebound in the DS group. The bromocriptine doses used (5 mg daily for 2 weeks) caused no objective side effects and no subjective restraint. Treatment with bromocriptine caused no untoward effect on the blood clotting, while in the DS group a slower return to normal antithrombin III could be observed. Also in this group one case of thrombophlebitis occurred. Bromocriptine can be administered to puerperal women for the suppression of lactation.