We studied the ultrastructural changes in gilts' pineal gland after the oral administration of melatonin in dose 3 mg once daily (in ethanol vehicle) for 46 consecutive days. Appearance of the pinealocytes containing numerous secretory granules and an enlarged Golgi apparatus was the most visible effect of the melatonin administration. The melatonin treatment caused also some differences in the structure of pinealocytes with cytoplasmic dense bodies. The present results suggest that the orally administered melatonin had some effects on pigs' pineal gland.