The effect of ergonovine maleate on prolactin secretion and lactation was determined in 10 puerperal women. Serum prolactin concentration measured by radioimmunoassay resulted in 537.2 +/- 45.9 ng/ml (M +/- SE) before the oral administration of 0.6 mg ergonovine maleate, and 89.7 +/- 25.6 ng/ml after 7 days of therapy. The serum prolactin concentration seen in the control group of nonlactating women (562.0 +/- 36.1 and 218.0 +/- 27.3 ng/ml) was significantly greater than (P less than 0.01) that seen in the treated group. In 2 additional patients it was also demonstrated that the simultaneous administration of ergonovine by oral and intravenous routes potentiates the suppressive effect on prolactin secretion. Three of the 10 treated women showed progressive inhibition of lactation. This study shows that ergonovine maleate interferes with prolactin secretion, and may decrease milk production.