Aspects of corticosterone and aldosterone metabolism have been studied in the marsh frog Rana ridibunda Pallas, kept in breeding conditions at a temperature of 16 degrees C. The effects of hypophysectomy on corticosteroid dynamics were also assessed. Two methods, single injection and continuous infusion, were adapted to measure the Metabolic Clearance Rate (MCR), the Secretion Rate (SR) and the half-time (T1/2) of these two hormones, using 3H-corticosterone and 3H-aldosterone as tracers. 10 The two methods provided similar values for the metabolic clearance rate (MCR) when, after the single injection, the MCR was estimated by integrating the area under each disappearance curve of radioactive corticoid. Estimates of MCR by compartmental analysis were not satisfactory. 20 The method of continuous infusion, although carried out in anesthetized animals, is more satisfactory than the single injection in unanesthetized frogs, as the plasma corticosterone in the latter increases significantly during the measurements time. On the contrary, the corticosterone concentration is constant during the infusion, at a little higher than basal level. 30 When measured at the temperature of 20 degrees C, the metabolic clearance rate for corticosterone was higher than that for aldosterone (8.08 +/- 0.29 ml/min/kg versus 5.32 +/- 0.20 ml/min/kg). These values are higher than in fish and lower than in homeothermic vertebrates. 40 The confusion between the adrenal secretory rate and the total production rate, possible for aldosterone, is somewhat hazardous for corticosterone, but production of this steroid from peripheral precursors has not been proved. 50 Adenohypophysectomy reduces the metabolic clearance rate and the secretory rate for corticosterone as well as for aldosterone.