The possible involvement of nitric oxide in the prevention by morphine of apomorphine- and oxytocin-induced penile erection and yawning was investigated by measuring the concentration of NO2- and NO3- in the dialysate obtained with a vertical microdialysis probe implanted in the paraventricular nucleus of the hypothalamus of male rats. Either apomorphine (80 micrograms/kg s.c.) or oxytocin (30 ng i.c.v.) increased significantly basal NO2- and NO3- concentration in the paraventricular dialysate, penile erection and yawning. Morphine (1.5 and 10 mg/kg i.p.) prevented dose-dependently either apomorphine or oxytocin responses when given 15 min before apomorphine or oxytocin. Prevention by morphine of apomorphine and oxytocin responses was abolished by naloxone (3 mg/kg i.p.) given 15 min before morphine. Morphine prevented apomorphine and oxytocin responses also when given in the lateral ventricles or directly in the paraventricular nucleus. In contrast, the selective agonist of the kappa opioid receptor subtype U-69,593 was found to be ineffective. The present results confirm previous findings showing that morphine acts through mu receptors in the paraventricular nucleus to prevent apomorphine and oxytocin-induced penile erection and yawning and suggest that this morphine effect is mediated by a decreased activity of nitric oxide in the paraventricular nucleus of the hypothalamus.