A sensitive and specific radioimmunoassay for alpha-melanocyte-stimulating hormone (alpha-MSH) was developed. Extracts of the neurointermediate lobe of the rat produced displacement curves which were parallel to those obtained with synthetic alpha-MSH. The mean immunoreactive alpha-MSH concentration in neurointermediate lobes from normal adult rats was 2768 +/- 200 (S.E.M.) ng/lobe. This accounted for approximately 78% of the MSH activity of the neurointermediate lobe as measured by bioassay. Much lower levels of immunoreactive alpha-MSH were found in the anterior lobe of the rat. Extracts of rat serum and plasma also contained immunoreactive alpha-MSH and the mean level was found to be 237 +/- 20 pg/ml. This was slightly lower than the level measured in rat plasma by bioassay. Increased levels of alpha-MSH were found in plasma of rats 1 and 3 h after a single injection of trifluoperazine and after 1-5 min of ether anaesthesia. These changes were reflected by decreases in the alpha-MSH content of the neurointermediate lobe.