Pituitary responsiveness to GHRH (1-29) NH2 (GHRH, 5 μg/kg iv) was analysed under sodium pentobarbital anesthesia (50 mg/kg ip), on days 30 and 90 in male rats orchidectomized or sham-operated 7 days earlier. Other groups of rats were orchidectomized or shamoperated on day 23 and tested on days 30, 45, 60 and 90. In the sham-operated animals, GHRH stimulated GH secretion on day 90, but not on day 30. GHRH-induced secretion was similar on days 45, 60 and 90 in orchidectomized and sham-operated animals. Orchidectomy on day 83 reduced this GHRH-induced GH secretion on day 90. In contrast, orchidectomy on day 23 enhanced the pituitary responsiveness to GHRH a week later. These results suggest that the increase in pituitary responsiveness to GHRH with age is independent of the testicular function and that the effect of orchidectomy depends on both the age of the rats at orchidectomy and the time elapsed between the orchidectomy and the administration of GHRH.