Serotonergic control over the reflex oxytocin (OT) release was investigated in anesthetized rats and in conscious rats. The effects of drugs were tested in the first case, on the electrical activity of oxytocinergic cells during suckling and in the second case, on the litter weight gain after 30 min suckling (indirect index of OT release). In rats anesthetized with urethane (1.2 g/kg), intraventricular injection of 1 μg serotonin interrupted the regular pattern of the neurosecretory bursts and milk ejections for about 15-20 min (inhibitory effect of the ‘all-or-none’ type). 10 μg cy-proheptadine or R47465 (serotonergic antagonists) slightly but significantly decreased the mean delay between two neurosecretory bursts without modifying their amplitude and in half the cases, disturbed their periodicity with occasional appearance of very close dual neurosecretory bursts. Pretreatment with p-chlorophenylalanine (PCPA) (250 mg/kg i.p.) did not prevent or affect the regular milk ejection pattern. The inhibitory effect of 5-HT was lengthened by fluoxetine, a 5-HT reuptake inhibitor (1 μl of 10-<sup>4</sup> M solution into the 3rd ventricle) and prevented by 5 μg R47465. In conscious rats, all the above drugs had an opposite effect. 5-HT and 5-HTP (5-HT precursor) did not affect the milk ejection reflex, whereas serotonergic antagonists and PCPA had an inhibitory effect. Injecting 5-HTP into the PCPA-treat-ed mothers restored their ability to release OT in response to suckling. Hypotheses for these opposite effects are discussed.