Wolfson Laboratory for Research in Gerontology, Department of Zoology, University of Hull, Hull, HU6 7RX (Received 5 April 1911) Jones, Brush & Neame (1972) reported that the level of adrenocorticotrophin (ACTH) in the plasma returned to the control value as early as 10 min after sham adrenalectomy. In contrast, Cook, Kendall, Greer & Kramer (1973) observed no decline in the level of radioimmunoreactive ACTH in the plasma until 40 min after the onset of a 2·5 min ether stress. Furthermore, the stress levels of ACTH were maintained for 2 h in animals subjected to continual ether stress. Therefore, the levels of both ACTH and corticosterone in sequential samples of plasma from control and dexamethasone-treated rats were measured, in order to see whether a negative feedback mechanism, that acts on the pituitary gland as the level of corticosterone increases, operates even in the presence of continuous stress. Blood samples were taken