It has been shown that the adrenal cortex gives a diminished secretory response to exogenous corticotrophin (ACTH) 2\p=n-\3h after exposure to moderate doses of ionizing radiation (Ungar, Rosenfeld, Dorfman & Pincus, 1955;Shima & Matsuba, 1963). The present paper describes observations made on adult dogs for up to 14 days after X-irradiation of one adrenal gland.Adult mongrel dogs weighing 10\m=.\3\p=n-\18\m=.\5 kg were used. Under sodium pentobarbitone anaesthesia, the left adrenal gland was exposed by the left lumbar route and irradiated with 2000 R of X-rays. The X-ray characteristics were: 180 kV peak, 20 mA, focus\p=m-\surfacedistance 37 cm, filtration of 0\m=.\5mm Cu and 0\m=.\5mm Al, and half-value-layer 1\ m=. \ 0 mm Cu. Dose-rate to the adrenal gland was approximately 130 R/min. Observations were carried out at 2, 7 and 14 days after irradiation.On the day of observation, each animal was injected s.c. with 0\m=.\1 mg dexamethasone-21-phosphate/kg. One hour after this treatment, the animal was anaesthetized with an i.v. injection of 25 mg sodium pentobarbitone/kg, and the left lumboadrenal vein was approached and cannulated by the method of Satake, Sugawara & Watanabe (1927). After two samples of adrenal venous blood had been collected, ACTH (Organon) (5 mu./kg) was injected into the saphenous vein. Timed adrenal venous blood samples were then collected at 15, 30, 60, 90 and 120 min. One millilitre of the plasma from each sample was analysed for 17-hydroxycorticosteroids (17-OHCS) by the method of Porter & Silber (1950). The lower limit of detectability of 17-OHCS in 1 ml plasma was 0-1 /tg. The rate of adrenal 17-OHCS secretion estimated at 30 and 10 min before ACTH injection was, with a few exceptions, extremely low (Table 1). Control non-irradiated dogs which received 5 mu. ACTH/kg showed an increase in the secretion of adrenal 17-OHCS. The response was seen at 15 min after the injection, but the effect did not last up to 30 min since the secretion at that time had fallen to pre-injection levels. At 2 days after X-irradiation of the adrenal, 17-OHCS secretion in irradiated dogs which received ACTH increased markedly at 15 and 30 min, and at 60 min remained appreciably increased above the pre-injection levels. At 7 and 14 days after X-irradia¬ tion, irradiated dogs showed marked increases in 17-OHCS after the injection of ACTH. With a few exceptions, the increased rates were sustained up to 60 min, but a return to the pre-injection levels tended to occur by 90 min.