Increase of plasma prolactin in chronically ovariectomized rats by transplanting pituitary glands from male rats to the kidney capsule reduced pituitary LH output for 1 week. At the end of this time, despite the fact that prolactin remained high, plasma LH levels rose to reach the value seen in ovariectomized control rats. High plasma prolactin did not affect FSH concentrations. Treatment of ovariectomized rats with apomorphine, a dopamine-receptor stimulating drug, reduced pituitary LH release in a dose-dependent way at doses of 1, 2, 5 and 10 mg/kg body weight. Higher doses either had no effect or they stimulated pituitary LH release. Apomorphine did not reduce pituitary LH secretion in chronically hyperprolactinaemic rats at doses which significantly inhibited LH release in ovariectomized animals. Plasma FSH levels were not affected by apomorphine treatment. In both groups of animals the drug reduced plasma prolactin levels to basal values, indicating that the dopamine agonist inhibits pituitary prolactin release by direct action on the pituitary gland. These results suggest that dopamine inhibits pituitary LH release, probably by reducing hypothalamic LH-releasing hormone release. The mechanism by which chronically high prolactin levels cause apomorphine to be less effective in reducing pituitary LH release may be a desensitization of some dopamine-receptive mechanism which is inhibitory to LH release.