At 1 h after a single subcutaneous injection of haloperidol into early proestrous rats, there was an 11-fold elevation in serum prolactin. Pituitary prolactin concentration 4 h after administration of 50 µg haloperidol/100 g BW was significantly decreased (52% of control values). In male rats, a single subcutaneous injection of 50 µg haloperidol produced an approximately 4-fold increase in serum prolactin at the end of 1 h, and a significant reduction in pituitary prolactin concentration at the end of 4 h. Hypothalamic PIF activity in male rats was reduced significantly by haloperidol whenever serum prolactin levels were elevated. These results suggest that the increased release of pituitary prolactin by haloperidol is mediated by a decrease in hypothalamic PIF activity.

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