The disappearance of radioactivity after the injection of 14C-labeled-Pro-Leu-Gly-NH2, an MSH release-inhibiting factor (MIF-I), was measured in rats which had been subjected to various procedures. Delayed disappearance of the radioactivity to a small but statistically highly significant extent was found in hypophysectomized rats compared to rats with an intact pituitary. Pinealectomy and adrenalectomy did not change the half-time disappearance. Dehydration, hypothalamic destruction, or administration of excess saline, MSH, and large amounts of unlabeled MIF-I were all used as control procedures. These failed to demonstrate the mechanism by which the prolonged disappearance occurred, but did raise other questions. The half-time disappearance of tritiated inulin was not significantly different in hypophysectomized or intact rats. The results indicate that the absence of the pituitary gland prolongs the half-time disappearance of radioactivity from the blood of rats injected with labeled MIF-I.