Eight milligrams of prostaglandin E2 (PGE2; 2 mg every 30 min) were given orally to 12 puerperal women on day 4 after delivery. Plasma levels of prolactin showed a significant decrease in comparison with basal levels and with those of controls sampled over a similar period. The same dose of PGE2 was ineffective in modifying the plasma prolactin levels in four puerperal women on day 30 after delivery and in four non-pregnant women with normal levels of prolactin. It is suggested that a PGE2-induced decrease in prolactin could be mediated by hypothalamic dopaminergic neurones.