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The occurrence of circadian variations in the concentration of prolactin in the plasma of 6- to 9-month-old male rats has been assessed in animals exposed to light for 14 h/day (lights on 06.00–20.00 h). Blood samples were obtained after decapitation, or from individual rats at regular intervals via a permanent cannula. Care was taken to limit stress during sampling. The concentration of prolactin in the plasma was significantly lower between 07.00 and 15.00 h than at other times. Between 15.00 and 20.00 h (during the light period), the concentration of prolactin was significantly higher in comparison with the preceding period, or with the remainder of the 24 h period. During the night, the concentration fluctuated, probably because of episodic releases of the hormone. The possible physiological significance of a circadian rhythm in the plasma concentration of prolactin and the implications for endocrine experimentation are discussed briefly.