OBJECTIVE: To evaluate the circulating concentrations of the neuroactive steroids in response to an i.v. L-tryptophan (L-TP) challenge across the menstrual cycle in women with premenstrual syndrome (PMS) and in controls. METHOD: An i.v. L-TP challenge was administered eight times during 1 month to five women with prospectively documented PMS and five age- and body mass-matched controls. Progesterone, allopregnanolone pregnenolone and 3alpha-5alpha-tetrahydrocorticosterone were assessed 15 and 0 min before, and at 30, 60 and 90 min after the challenge, across the menstrual cycle. RESULTS: In response to L-TP challenge, only allopregnanolone concentrations were significantly increased across the cycle and this increase was of a greater magnitude in women with PMS. Pregnenolone and 3alpha-5alpha-tetrahydrocorticosterone concentrations were not affected in women with PMS or controls after L-TP challenge. CONCLUSIONS: The data provide evidence for possible interaction between the serotonergic system and the neuroactive steroid, allopregnanolone. Women with PMS demonstrated a more significant increase in allopregnanolone concentrations in response to L-TP challenge, which could be due to an initial low basal serotonergic tone in the luteal phase in the PMS group.