We study the cellular processes by which steroid hormones act in neurons, particularly with respect to their involvement in behaviors. During the estrous cycle of female rats, mice and other rodent species, the ovarian hormones, estradiol and progesterone, regulate the expression of reproductive and many other behaviors. The sensitivity of specific neurons to each of the hormones is determined in part by the concentrations of hormone-specific intracellular receptors. These receptors are essential in mediating the effects of steroid hormones on some behaviors, possibly by modulating gene transcription and synthesis of specific proteins. In our recent work, we have found that exposure to particular stressors, including immune challenge, during puberty permanently alters the response of the brain to ovarian hormones. This in turn results in dramatic changes in behavioral response to the hormones in adulthood.