Groups of normal male rats were bled at 6 PM just before turning out the lights and at different times during the night. Plasma prolactin as determined by radioimmunoassay was unchanged when the second blood sample was taken at approximately 2 AM, began to rise slightly at 3 and was markedly elevated by 5 AM. Even if the lights were left on in the animal room during the night for one night, the 4 AM elevation persisted. The elevations observed in normal rats between 3:30 and 5 AM, whether the lights were off or on, were absent in animals which had been pinealectomized. There was no significant change in the initial 6 PM value of plasma prolactin in pinealectomized animals on comparison with the values observed in normal rats. It is suggested that the early morning rise in plasma prolactin in male rats is mediated by enhanced release of pineal hormones.
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