The mechanism of the observed synergistic effect of prolactin and androgen on the lateral lobe of the rat prostate is not established. The observation that prolactin alone delayed the rate of loss of weight, protein, and DNA of the lateral lobe in castrated rats has led us to question the assumption that the effect of prolactin is produced by a modification of recognized androgen-induced intracellular changes. The present study was conducted to explore whether or not the sites of prolactin action in the rat prostate coincided with those recognized as the androgen effect. Two anterior pituitaries from female donors were grafted under the right renal capsule of adult male Sprague-Dawley rats. Seven days later, bilateral orchiectomy and unilateral nephrectomy were performed in these rats. In one half of the animals, the kidney bearing the pituitary grafts was removed. In the other half, the contralateral kidney was removed. Seven days following the orchiectomy-nephrectomy, animals bearing the pituitary grafts had a higher level of serum prolactin (93 +/- 7 ng/ml, mean +/- SE) than in those without the graft (26 +/- 3 ng/ml). This condition of hyperprolactinemia was associated with the delay of castration-induced regression in the lateral prostate. The rate of protein degradation, as judged by the amount of radioactivity remaining in the tissue following a single i.v. pulse of 3H-leucine 24 hr before orchiectomy-nephrectomy, was significantly slower in the lateral prostate in graft-bearing animals than in those without grafts. However, the rate of protein synthesis, as judged by both in vitro and in vivo incorporation of 3H-leucine into the protein fractions of the prostate, was not significantly different between the two groups. Nor was the total activity of cathespin D in the lateral prostate different. These results were not consistent with parameters known as the effect mediated through androgen-controlled mechanisms. We therefore conclude that the action of prolactin in the regressing rat prostate is probably not mediated through the action of androgen.