The effect of chronic noise stress on the response of anterior pituitary hormones to the same or to another stressor (forced swimming) was studied in adult male Wistar rats. Both acute stressors increased corticosterone, prolactin, LH and TSH secretion and inhibited GH secretion. Previous chronic exposure to noise reduced corticosterone response to the same stimulus without modifying corticosterone response to a novel acute stress. Neither prolactin nor TSH responses to acute noise were reduced by previous chronic exposure to noise. Since chronic noise increased basal levels of LH and decreased those of GH, the response of these hormones to acute stress was expressed as percent changes of their respective basal values. It was found that chronically stressed rats showed diminished LH response to noise but not to forced swimming. GH showed the same pattern without reaching statistical significance. These data indicate that the response of some anterior pituitary hormones can adapt after repeated exposure to the same stressor. When adaptation occurred, this was specific for the stressor which the animals were repeatedly exposed to. The pituitary-adrenal axis appears to be the most reliable index of adaptation to chronic stress among all the anterior pituitary endocrine axes.