Radioactive synthetic alpha-MSH, prepared by selective tritiation of its dibromo-L-tyrosine-derivative, was injected into the carotid artery of intact, hypophysectomized or pinealectomized rats. More radioactivity was found in the occipital cortex, cerebellum, and pons-medulla than in most other brain parts of rats decapitated 15 sec and, to a less significant extent, 30 min after injection. Part of this radioactivity behaved like alpha-MSH in several identification procedures. After administration of 3H-tyrosine, the least radioactivity was consistently found in the pons-medulla. Large amounts of radioactivity after 3H-alpha-MSH were found in the pituitary and, particularly in the hypophysectomized rats, in the pineal. The half-time disappearance of the first component of radioactivity from the blood of each group of rats injected with tritiated-alpha-MSH was no more than 2.5 minutes, a time considerably shorter than the behavioral and EEG effects of this pituitary hormone on visual attention.