The influence of lisuride, a new semisynthetic ergot derivative, was investigated in women during the puerperium. Treatment with 600 or 900 micrograms lisuride over 14 days (each test group n = 25) caused an immediate drop of elevated prolactin (PRL) levels in all patients in comparison to values seen in normal, nonpregnant women (less than 30 ng/ml). Such a sharp decline was not seen in two control groups of puerperal patients, nursing as well as nonnursing, placebo-treated women. The clinical efficacy in preventing or suppressing lactation was clear cut and comparable to the known PRL lowering effect of bromocryptine. Severe side effects were not observed during lisuride treatment with these dosages. The postsuckling PRL increase was abolished by a single oral dose of lisuride (100, 200, 300 micrograms), similar to that seen with bromocrytine (2.5 mg). The inhibition was dose dependent. Only doses of 200 and 300 micrograms showed, in comparison to a placebo group, a significant long-lasting (greater than 8 h) suppression of PRL secretion.