SUMMARY: The distribution and disappearance of radioactivity after the intravenous administration of [1,2-3H]corticosterone and [1,2-3H]aldosterone to the pigeon were studied according to one- and two-compartment model systems. The volume of distribution (AVD) of the total and methylene chlorideextractable radioactivity in the plasma of hypophysectomized pigeons injected with labelled corticosterone was significantly increased. Also the biological half-life (T½) of each radioactive fraction in plasma was significantly prolonged. No significant change in the metabolic clearance rate (MCR) of labelled corticosterone was observed in the hypophysectomized pigeon. The AVD of the total and methylene chloride-extractable tritium in the plasma of the hypophysectomized pigeon injected with labelled aldosterone was unchanged but the T½ of both plasma fractions was significantly prolonged. The estimated MCR of labelled aldosterone was significantly diminished. The concentration of total fluorogenic corticosteroid in plasma from the chronically hypophysectomized pigeon was reduced to one-third of that in the sham-operated bird, and corticotrophin-replacement therapy restored the concentration to normal. After hypophysectomy, the plasma corticosterone concentration was reduced to 20% of the concentration in the sham-operated birds, and replacement therapy with corticotrophin again restored the plasma levels to normal. Estimation of the endogenous corticosterone secretory rates indicated that chronic hypophysectomy was accompanied by a decline commensurate with the reduction in the plasma corticosterone concentration.