A superfusion technique was employed in the study of the release of dopamine (DA) and norepinephrine (NE) from medial basal hypothalamic fragments. The DA and NE collected in the superfusion fluid were quantified by a radioenzymatic assay. The amounts of DA and NE released by the medial basal hypothalamic fragments were found to be dependent upon the CA2+ and K+ concentrations in the superfusion fluid. The effect of PRL on the amounts of DA and NE released during exposure to a submaximal stimulus of 30 mM K+ was evaluated. PRL in concentrations of 50-5000 ng/ml augmented the K+-induced release of DA and NE in a concentration-dependent manner. The PRL augmentation of the release of both DA and NE was prevented by the addition of anti-PRL gamma-globulin to the superfusion medium. In view of the inhibitory effect of DA on PRL secretion, these findings are consistent with the conclusion that PRL can influence its own secretion by stimulating the release of hypothalamic DA. The observation that PRL can also augment the release of NE is supportive of the view that PRL can influence the secretion of nondopaminergic neurotransmitters within the hypothalamus.