An assay for corticotrophin releasing factor (CRF) is described, using plasma corticosterone responses in ACTH-primed ‘grafted’ rats (hypophysectomized rats with multiple ectopic pituitary grafts). Pooled acid extracts of rat stalk median eminences (SME) were used as a local CRF standard. A satisfactory dose response relationship exists for intravenous doses of SME between 0.2 and 1.0 SME and the index of precision ( λ) is approximately 0.2. The ACTH (0.22 mU (1962)/SME) and vasopressin content (<5 mU/SME) of rat hypothalamic extracts make a negligible contribution to the assay responses. No potency difference was found between frozen and freeze-dried extracts of pooled rat SME.