Luteinizing hormone releasing hormone (LHRH) administered to ovariectomized marmosets treated with a low dose of 17β-oestradiol significantly enhanced proceptivity within 2 h. Without oestradiol priming LHRH treatment did not produce this effect. The dose of LHRH given did not significantly alter plasma levels of cortisol or progesterone. The behavioural effect of this treatment is therefore likely to have been mediated via a direct central action rather than indirectly through stimulating adrenocortical steroid secretion. These results provide the first demonstration that LHRH can potentiate the proceptive behaviour of a female primate