The effects of single and repeated electroconvulsive shock (ECS) on the hypothalamic-pituitary-adrenal (HPA) axis and plasma catecholamines were studied. Rats were divided into three groups and each group received sham treatment, single ECS, or ten once-daily ECS. Jugular venous blood samples were obtained immediately before treatment and at 10, 30, 60, and 90 min following sham treatment, a single ECS or following the last of ten ECS. Plasma concentrations of corticosterone (CS), ACTH, immunoreactive beta-endorphin 0r-BE), epinephrine (E) and norepinephrine (NE) were determined. Following the single ECS plasma CS was significantly elevated at 10 and 30 min, ACTH was significantly elevated at 10, 30, and 60 min, whereas ir-BE and E peaked at 10 min and returned to basal concentration by 30 min. The concentration of plasma NE did not significantly vary at any time point. Following the tenth ECS the concentration of plasma CS revealed a significant attenuation of the increase at 10 and 30 min when compared with the CS changes observed following a single ECS. Plasma ACTH following chronic ECS was also significantly decreased in magnitude at 10, 30, and 60 min when compared with plasma ACTH levels following a single ECS. It-BE in plasma following ten ECS mirrored the changes following single ECS. In contrast to the attenuation of CS and ACTH following chronic ECS, the increase in peripheral catecholamines was markedly elevated after the last of ten ECS. Compared with single ECS, ten ECS produced significant increases in plasma E at 10, 30, and 60 min and at 10, 30, 60, and at 90 min for NE. The results of this study indicate that, upon repeated ECS there is an attenuation of the elevation of plasma ACTH and CS, and an enhancement of E and NE responses in comparison with those observed following a single ECS.