In order to identify prolactin regulating factors, the effect of various neuropeptides on prolactin secretion by the adenohypophysis has been tested. 1 degree Histidyl-proline-diketopiperazine (DKP), a major degradation product of TRH in hypothalamus and pituitary, inhibited prolactin secretion from incubated hemipituitaries (Fig. 1) with an apparent affinity of 0.5 nM. Histidyl-prolineamide and histidyl-proline, other degradation products of TRH, had no effect. TSH secretion was not affected under the same conditions. 2 degrees Vasoactive intestinal peptide (VIP) stimulated prolactin secretion in vitro in a dose dependent manner. The secretion of other adenohypophyseal hormones was not affected. This effect is not mediated by a dopaminergic mechanism, since it was not blocked by neuroleptics (Table I). 3 degrees Morphinomimetic peptides had no effect on prolactin secretion in vitro, but blocked the dopamine inhibition of prolactin secretion. The effect of metenkephalin and beta-endorphin was dose dependent and was blocked by naloxone (Fig. 2 and 3). Thse results indicate that specific receptors to various neuropeptides seem to be present on prolactin cells.