We study the cellular processes by which steroid hormones act in neurons, particularly with respect to their involvement in reproductive behavior. During the estrous cycle of female rats and other rodent species, the ovarian hormones, estradiol and progesterone, regulate the expression of reproductive behaviors. The sensitivity of specific neurons to each of the hormones is determined in part by the concentrations of hormone-specific intracellular receptors. Intracellular steroid hormone receptors are essential in mediating the effects of steroid hormones on some behaviors, possibly by modulating gene transcription and synthesis of specific proteins.