Maximal incremental changes in plasma LH were compared in adult hens and cockerels after i.v. injection of chicken (c) LHRH-I (pGlu1-His2-Trp3-Ser4-Tyr5-Gly6-Leu7-Gln8-Pro9-Gly10-NH2) or cLHRH-II (pGlu1-His2-Trp3-Ser4-His5-Gly6-Trp7-Tyr8-Pro9-Gly10-NH2) The LH response to cLHRH-I and -II was more rapid and greater in cockerels than in hens. The potencies of the two decapeptides were the same in cockerels but different in hens. Relative to cLHRH-I, the potency of cLHRH-II was 0.91 (0.6-1.2; 95% confidence limits) in cockerels and 36.5 (16.8-128.6) in hens. The greater potency of cLHRH-II relative to cLHRH-I in the hen than in the cockerel could not be accounted for by sex differences in the half-lives of the decapeptides in the peripheral circulation. The half-lives of both decapeptides in hens and cockerels ranged between 2.42 and 3.77 min. It is concluded that the interaction between LHRH-I and -II and the gonadotrophs is sexually differentiated in the domestic fowl. A new homologous radioimmunoassay was establishyed for cLH. As in other chicken LH radioimmunoassays, there was evidence of cross-reactivity with TSH.