Three experiments were conducted to determine 1) the relationship between prolactin and growth hormone (GH) secretion in mares and the response to GH-releasing hormone (GHRH), 2) whether plasma GH and prolactin concentrations differed among mares, stallions, and geldings, and 3) whether sexual differences existed after administration of GHRH and acute exercise. In Exp. 1, 10-min blood samples were collected from 12 mares for 8 h, and GHRH (0, 45, 90, or 180 micrograms) was administered at 6 h. In Exp. 2, 15-min blood samples were collected for 4 h from 10 mares, stallions, and geldings. In Exp. 3, eight horses of each sexual status were administered GHRH at 0900; later that day, each horse was exercised for 5 min. Blood samples were collected every 10 min around each event. In Exp. 1, prolactin concentrations decreased (P < .01) over the 8-h period, and there was an average of 2.9 +/- .5 episodes of increased secretion during that time; there was no correlation between these episodes and those in GH secretion. Prolactin concentrations were not affected (P > .1) by GHRH. In Exp. 2, average concentrations of GH were 2.4, 8.6, and 8.5 ng/mL for mares, stallions, and geldings, respectively; males differed from females (P < .05). Stallions and geldings had more (P < .05) peaks in GH concentrations and greater (P < .05) amplitude of peaks than mares. In contrast, prolactin concentrations were greater (P < .02) in mares and stallions than in geldings. In Exp. 3, GH response to GHRH was greater (P < .03) in stallions than in mares or geldings.(ABSTRACT TRUNCATED AT 250 WORDS)