High serum prolactin levels in ovariectomized rats were induced by transplantation of additional pituitaries under the kidney capsule. Such high prolactin levels reduced serum LH but not FSH levels 3 days after pituitary transplantion. LH and FSH values were at a control levels at day 15 and above these levels at day 24, although prolactin values were still high. Dopamine (DA) and norepinephrine (NE) turnover in the anterior part of the mediobasal hypothalamus (AMBH) and in the medial preoptic area (MPO) was measured by following the decrease in NE or DA content after synthesis blockage with a-methyl-p-tyrosine. The content was measured using a radioenzymatic assay. DA turnover in the AMBH was significantly increased 3, 15 and 24 days after induction of hyperprolactinemia. Although NE concentration in the AMBH was also increased at these times an increased NE turnover was statistically significant only at day 24. DA as well as NE turnover rates in the MPO were reduced by day 3 after pituitary transplantation and at control values at day 15 and 24. It is concluded that the increased DA turnover in the AMBH depresses pituitary LH release probably by inhibiting hypothalamic LH-RH secretion. This inhibition is counteracted by the somewhat slower increase of NE turnover resulting in normalization of LH levels. The increased gonadotropin levels after long-lasting hyperprolactinemia may be due to desensitization of the DA receptor, which was reported earlier. The reduced NE turnover in the MPO 3 days after induction of hyperprolactinemia may be an additional factor in reducing pituitary LH release acting at the level of LH-RH-producing perikarya.