Resting and stress-induced plasma corticosterone concentrations in adult male hypophysectomized rats bearing a pituitary homograft in the hypothalamus were determined fluorimetrically. Plasma corticosterone resting levels were maintained and a significant elevation of these levels in response to stress occurred only in those rats in which the graft was in the medial basal hypothalamus (hypophysiotrophic area, HTA) and the implant was connected with the median eminence. If these latter connections of the graft in the HTA did not exist, resting plasma corticosterone concentrations were reduced and the applied stress did not induce any elevation of the low pre-stress levels. On the basis of the present and previous findings, it is assumed that the site of CRF production is the median eminence region.