The efficacy and potency of 14 GH-releasing factor (GRF) analogs, substituted in position 1 to 7, on adenylate cyclase activation in crude homogenates from rat anterior pituitary were related to those of human pancreatic GRF(1-29)-amide and vasoactive intestinal peptide. Among several D-amino acid substitutions, that in position 2 was the only one to yield a super-agonist [with a Kact (concentration required for half-maximal adenylate cyclase activation) 2 times lower than that of GRF(1-29)-NH2]. By contrast, D-isomer substitution in position 1 and 3 was without effect and D-isomer substitution in position 4, 6, or 7 decreased the affinity of the analog. The N-acetylated analog of GRF was as potent and active as the parent peptide, and the identity of the amino acid in position 2 of (N-Ac-Tyr1)-GRF(1-29)-NH2 proved to be determining for enzyme activation, with D-Phe2 and D-Trp2 derivatives acting as partial agonists and the (N-Ac-Tyr1,D-Arg2) analog being an efficient competitive antagonist of GRF(1-29)-NH2. With use of this antagonist, it was possible to demonstrate that GRF and vasoactive intestinal peptide receptors represent distinct entities in the rat anterior pituitary.