The influence of the anterior pituitary on tissue CRF was investigated in two sets of experiments using lesioned, hypophysectomized, adrenalectomized donor rats. Donors were injected with 1 or 0.5 anterior pituitary equivalents 3 h before transfer of plasma to recipient animals. Injection of 1 pituitary equivalent significantly reduced levels of corticosterone in recipient rats compared to saline injection at 3 different time intervals following the transfer. In a second series of experiments donor animals received replacement with saline, ACTH, TSH, or PRL at 0, 2 and 4 h following adrenalectomy; transfer of plasma to recipient animals was at 5 h. Of the three hormones injected only ACTH significantly reduced tissue CRF activity in donor animals. Recipients of these donors showed suppressed levels of corticosterone compared to recipient animals whose donors were injected with saline, TSH or PRL. The ACTH dose-response curve indicates that the effective dose for suppression of tissue CRF in donor animals is in the range of 1–10 mU/ml. Results of these experiments clearly show that tissue CRF is inhibited by the anterior pituitary hormone ACTH rather than by elevated levels of corticosterone. These experiments suggest that feedback regulation of tissue CRF release by ACTH may occur in response to prolonged physical stres