Abstract:Binding of3H‐enkephalinamide and3H‐naloxone to P2‐fractions of whole rat brain homogenate displayed saturable, stereospecific binding to receptor sites with at least two binding sites for3H‐met‐enkephalinamide (type I: KD= 0.4 nM and Bmax= 35 fmol/mg protein; type II: KD= 5.7 nM and Bmax= 57 fmol/mg protein) and for3H‐naloxone (type I: KD= 1.5 nM and Bmax= 40 fmol/mg protein; type II: KD= 51 nM and Bmax= 255 fmol/mg protein), β‐endorphin and met‐ and leu‐enkephalin produced a concentration‐dependent inhibition of3H‐met‐enkephalinamide and3H‐naloxone binding with dissociation constants in the nanomolar range, but with very different displacement curves. Purified porcine ACTH (1–39) displaced both3H‐met‐enkephalinamide and3H‐naloxone with dissociation constants of 3.4 × 10‐ Mand 1.8 × 10−6M, respectively. The synthetic congeners, ACTH (1–32) and to a lesser extent ACTH (1–28) and ACTH (1–24) showed a similar effect, whereas other fragments of ACTH were inactive in concentrations ranging from 10−10to 10−6M. In the same concentration range cholecystokinin congeners (CCK‐8 and CCK‐4) were without effect. Since ACTH immunoreactive nerves seem also to contain β‐endorphin and furthermore, to show a partially overlapping distribution with the enkephalinergic systems it is possible that the binding of ACTH fr