BACKGROUND Transcranial magnetic stimulation is a relatively new technique for inducing small, localized, and reversible changes in living brain tissue. Although transcranial magnetic stimulation generally results in no immediate changes in plasma corticosterone, prolactin, and testosterone, it normalizes the dexamethasone suppression test in some depressed subjects and has been shown to attenuate stress-induced increases in adrenocorticotropic hormone in rats. METHODS In this study, serum corticosterone and testosterone concentrations were assayed in male rats immediately and 3, 6, 9, 12, 24, and 48 hours following a single transcranial magnetic stimulation or sham application. Serum prolactin concentrations were determined immediately and 2 hours following a one-time application of either transcranial magnetic stimulation or sham. RESULTS Transcranial magnetic stimulation animals displayed significantly lower corticosterone concentrations at 6 and 24 hours following a single application compared with sham-control values. Transcranial magnetic stimulation also resulted in lower corticosterone concentrations numerically but not statistically in transcranial magnetic stimulation animals immediately after application (p =.089). No significant differences were found between groups for serum prolactin or testosterone levels at any given collection time point. CONCLUSIONS These findings 1) suggest that transcranial magnetic stimulation alters the hypothalamic-pituitary-adrenal stress axis and 2) provide time-course data for the implications of the hormonal mechanism that may be involved in the actions of transcranial magnetic stimulation.