Female Wistar rats were divided into four groups. Group 1: The rats were pinealectomized by the Kuszak and Robin modified method or sham-operated 3 weeks after castration. They were injected with estrogen-progesterone 1 week after the pinealectomy and sacrificed by decapitation 48 hours after the injection. Group 2: The rats were pinealectomized or sham-operated after 1 week of pregnancy and decapitated after 20 days of pregnancy. Group 3: The rats were pinealectomized or sham-operated after

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... 19 days of pregnancy. The litter was separated from the mother immediately after delivery. These non-lactating rats were sacrificed by decapitation 3 days after delivery. Group 4: Pinealectomy and sham-operation were done by the same method as in group 3. The litter was not separated and allowed to suckle. These lactating rats were decapitated 3 days after delivery. Prolactin levels in the pituitaries and serum were determined by radioimmunoassay . We found that pinealectomy made the serum prolactin level significantly increase in pregnant and puerperal rats, and significantly decrease in castrated rats. The hypophysial prolactin level decreased in all groups, but there was no significant difference apart from group 3. These results indicate that the suckling stimulation made the prolactin secretion significantly increase even though the rats had undergone a pinealectomy. The pineal gland effects the hypothalamus appearing to inhibite PIF and PRF. This would suggest that the prolactin release action of the pineal gland has no relation to the route of the central nervous system concerning suckling stimulation.