Lesions of the suprachiasmatic nuclei (SCN) completely eliminated phasic LH release in ovariectomized rats as measured by the positive feedback response to estradiol benzoate (EB)/progesterone or the response to mating. Basal LH levels and the negative feedback response to EB were not affected. Lesions of the medial preoptic area (MPOA) or bed nucleus-dorsal MPOA also inhibited phasic LH release in ovariectomized rats as measured by positive feedback. However, the inhibition was not complete if there was no damage to the SCN, and the degree of inhibition was correlated with the size ofthe lesion. Basal LH levels and negative feedback were not significant affected. It is suggested that both the SCN and MPOA are involved in phasic LH release, the former in its role as a neural regulator of circadian rhythms and the latter as part of a diffuse system possibly including estrogen-sensitive and/or LH-RH neurons. Sexual behavior (lordosis) in hormone-primed, ovariectomized rats was not significantly affected by lesions of any structure in the suprachiasmatic-preoptic region.