In vitro, the prolactin response of perifused anterior pituitary cells of horses to thyrotrophin-releasing hormone (TRH) (0.1, 1.0, 10 and 100 nM), correlated significantly with hours of daylight (P less than 0.01). Baseline concentrations of prolactin also were significantly correlated with daylength (P less than 0.01). When response and baseline data were fitted by nonlinear least squares to a cosine function, the circannual phase was -0.06072 (+/- 0.02170) and -0.05560 (+/- 0.0255), respectively, which are not significantly different from that of daylength. In vivo, prolactin secretion was measured in jugular plasma and pituitary venous effluent. Prolactin secretion by the pituitary (20- to 30-sec samples) showed pulses of 2-7 min duration above a varying baseline. Jugular prolactin concentrations correlated significantly with daylength (P less than 0.01, n = 20). The fractional plasma clearance rate was 2.50 x 10(-2) and 3.31 x 10(-2)/min in 2 horses. Seasonal changes in prolactin secretion in vivo and in vitro may reflect the amount of prolactin available for release.