The effects of tobacco-smoke inhalation on a readily quantifiable neuroendocrine reflex, i.e., prolactin release in response to suckling, has been examined in lactating albino rats. After isolation from their pups for 8 hr, mothers exposed to puffs of smoke for 30 min released significantly less prolactin (P 0.01 or less) at 15, 30, 45, 60, and 90 min after suckling began than did air-exposed controls. At 120 min postsuckling, prolactin levels in smoke-exposed animals exceeded those of controls, perhaps reflecting prolactin accumulated in the pituitary during the time its release was inhibited. These results show that tobacco-smoke inhalation delays the suckling induced release of prolactin and provide the first evidence that smoke inhalation (nicotine?) can interfere with neuroendocrine reflexes associated with reproduction.