: Up till now the results of performed investigations suggest the involvement of opioids in the regulation of the pineal gland activity in mammals. On the other hand, they show the existence of large interspecies differences in the presence of opioid peptides in the pineal gland and in the effects of opiates on the melatonin secretion. The aim of the present work was to study the influence of morphine on the melatonin secretion in the domestic pig. Morphine (about 2.5 mg/kg) was given intravenously to immature gilts during the day, during the night or during the night with turned on fluorescent illumination (intensity 500 lx at the level of the animal heads) and plasma melatonin level was measured. The effect of various concentrations of morphine on basal or norepinephrine-stimulated melatonin secretion was also investigated using perfusion culture of pineal glands of immature female pigs. Morphine did not change plasma melatonin concentration in the domestic pig when administered in a single dose at the beginning of the light or the dark phase of the diurnal light-dark cycle. However, morphine administration at the beginning of the night resulted in significantly decreased the plasma melatonin level in animals exposed to light, with intensity 500 lx, which was insufficient to block nocturnal rise in plasma concentration of this pineal hormone in the untreated pigs. Morphine had also no effect on the level of basal and norepinephrine-stimulated melatonin secretion in vitro. The obtained results suggest that in immature pigs morphine does not influence directly the pineal activity, but may modulate the melatonin secretion indirectly, increasing the sensitivity of the system generating melatonin synthesis and secretion to the light.