A perifusion system was developed to investigate the control of alpha-melanocyte-stimulating hormone (alpha-MSH) release from rat brain. Hypothalamic slices were perifused with Krebs-Ringer bicarbonate (KRB) medium supplemented with glucose, bacitracin and bovine serum albumin. Fractions were set apart every 3 min and alpha-MSH levels were measured by means of a specific and sensitive radioimmunoassay method. Hypothalamic tissue in normal KRB medium released alpha-MSH at a constant rate corresponding to 0.1% of the total hypothalamic content per 3 min. The basal release was not altered by Ca2+ omission in the medium or addition of the sodium channel blocker tetrodotoxin (TTX). Depolarizing agents such as potassium (50 mM) and veratridine (50 microM), which is known to increase Na+ conductance, significantly stimulated alpha-MSH release in a Ca2+-dependent manner. When Na+-channels were blocked by TTX (0.5 microM) the stimulatory effect of veratridine was completely abolished whereas the K+-evoked release was unaffected. These findings suggest that: voltage-dependent sodium channels are present on alpha-MSH hypothalamic neurons; depolarization by K+ induces a marked stimulation of alpha-MSH release; K+- and veratridine-evoke releases are calcium-dependent. Altogether, these data provide evidence for a neurotransmitter or neuromodulator role for alpha-MSH in rat hypothalamus.