Immunocytochemical studies have identified immunoreactive prolactin (IR-PRL) in the hypothalamus and other areas of the rat brain. However, neither the release of IR-PRL from the hypothalamus nor its subcellular localization have been demonstrated. In this study, the release of IR-PRL from hypothalami obtained from female rats was examined using hypothalamic units incubated in vitro in Krebs-Ringer bicarbonate-glucose buffer. Hypothalamic tissue spontaneously released IR-PRL, and this release was increased by depolarizing concentrations of potassium by a calcium-dependent mechanism. Hypothalamic IR-PRL was also released from hypothalamic tissue obtained from hypophysectomized rats (14 days). The subcellular localization of IR-PRL was investigated using equilibrium-density centrifugation. Tissue homogenates from intact or hypophysectomized rats were centrifuged at 150 g at 4 °C for 10 min, and the supernatants were layered onto continuous sucrose gradients (1.00–1.27 g/ml) and centrifuged at 100,000 g (max.) for 16 h. IR-PRL in pituitary supernatants showed a high equilibrium-density peak with a modal density of 1.23 g/ml. Fractionation of the supernatant from ventral or dorsal hypothalamic tissue resulted in two high-equilibrium density peaks, a primary peak with a modal density of 1.23 g/ml and a smaller peak with a modal density of 1.10 g/ml. Both high-density peaks were maintained in tissue obtained from hypophysectomized rats and were disrupted by homogenization in hypo-osmotic medium. Together, these data suggest that hypothalamic IR-PRL is stored in membrane-bound particles which have densities similar to those of secretory granules and is released by a calcium-dependent mechanism when the tissue is depolariz