alpha MSH is known to act on several nonmelanocyte cell types, which has led to recent interest in its regulatory roles in fever, inflammation, immunity, and behavior. To determine its possible sites of action, we examined the distribution of alpha MSH receptors in a variety of tissues in mice and rats. The superpotent and enzymatically resistant alpha MSH analog, Nle4,D-Phe7-alpha MSH (NDP-MSH), was radioiodinated using lactoperoxidase (Enzymobeads) and purified by reverse phase chromatography for use as a tracer. [125I]NDP-MSH exhibited consistent and specific binding to cultured B16 and Cloudman S91 murine melanoma cells, which are highly responsive to alpha MSH. The tracer had full biological activity, as determined by its potency in stimulating melanogenesis in B16 cells. To study receptor distribution in vivo, [125I]NDP-MSH was administered iv to C3H/HeJ (pigmented) mice and Sprague-Dawley (albino) rats. To determine the specificity of tracer uptake by tissues, some animals received a large molar excess of alpha MSH together with [125I]NDP-MSH. Data were expressed as tissue to plasma radioactivity concentration ratios. In mice, specific (i.e. alpha MSH-inhibitable) binding of [125I]NDP-MSH was found in a number of glandular organs, including lacrimal, Harderian, preputial, submandibular, and adrenal glands and pancreas, as well as in brown and white adipose tissues, bladder, duodenum, skin, spleen, and hypothalamus. In rats, results were generally similar; specific tracer uptake was observed in lacrimal, Harderian, preputial, and thyroid glands; pancreas, duodenum, spleen, hypothalamus; and white adipose tissue. These results show that specific receptors for alpha MSH are widely distributed, suggesting that alpha MSH may affect the functions of a number of organs.