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Four experiments assessed factors affecting prolactin responses to sulpiride administration in horses. Experiment 1 compared the efficacy of the (−) enantiomer of sulpiride to that of the commonly used (+/−) racemic mixture. Mares were used in an 8 × 8 Latin square to compare the prolactin responses to four doses of levosulpiride to four corresponding doses of the racemic mixture at twice the dose. Responses at each dose indicated equal and similar (P > .1) responses. Experiment 2 compared the efficacy of 1 gram of orally administered racemic sulpiride to 100 mg of intramuscularly injected sulpiride in oil in mares primed with 50 mg of estradiol cypionate (ECP). Prolactin responses in groups receiving sulpiride were robust but similar in magnitude with minor differences in timing. In experiment 3, ECP-primed geldings received subcutaneous injections of 1.8 grams racemic sulpiride in vegetable shortening in one of three sites: the neck, the back below the withers, or the lower girth region; control geldings received no sulpiride. Prolactin responses to sulpiride lasted a minimum of 96 hours. In experiment 4, prolactin responses to 3 g of racemic sulpiride in vegetable shortening were compared to similar injections (3 g) in 5 mL of sucrose acetate isobutyrate (SAIB; SucroMate) or just SAIB (control) in ECP-primed geldings. Controls had no prolactin response to SucroMate, whereas both treatment groups had extended prolactin responses lasting at least 10 days. It is concluded that prolactin responses to sulpiride in horses can be greatly extended by using hydrophobic vehicles like vegetable shortening or SAIB.The antidopaminergic activity of the (−) enantiomer of sulpiride by weight on prolactin secretion is approximately twice that of the (+/−)-racemic mixture, confirming that the (−) enantiomer is the active component of the racemic mixture.Sulpiride administered orally had approximately one-tenth the antidopaminergic activity of sulpiride administered intramuscularly in oil.Administration of 1.8 grams of sulpiride in vegetable shortening to estrogen-primed geldings extended the stimulatory effect on prolactin secretion to a minimum of 96 hours.Intramuscular administration of 3 grams of sulpiride in sucrose acetate isobutyrate vehicle, in the form of SucroMate, extended the stimulatory effect on prolactin secretion to approximately 10 days.Prolactin responses to sulpiride in horses can be greatly extended by using hydrophobic vehicles like vegetable shortening or sucrose acetate isobutyrate.