The effect of chronic stress on basal and stress-induced alterations of TSH and GH was studied in adult male rats. Chronically stressed rats were subjected 6 days per week for 4 weeks to several acute stressors including saline injections, noise, ether and forced swimming. Each day, one stressor was chosen randomly. Twenty hr after the last stress session, basal levels of TSH were normal or increased, with no altered pituitary response to TRH. In contrast, the TSH rise induced by acute stress was blunted in chronically stressed rats. Chronic stress resulted in lower basal and acute stress levels of GH. These modifications were probably due to changes in the release of hypothalamic regulatory hormones, because no evidence for altered TSH response to TRH, and GH response either to GHRH or to somatostatin, was found. Some abnormal responses of GH to TRH and of TSH to GHRH were observed in chronically stressed rats. These data indicate that this type of chronic stress induced significant changes in basal and acute stress levels of GH and TSH in the rat. © 1987.