The basal body temperature (BBT) curve and the estimated time of ovulation, defined by the onset of gonadotropin preovulatory discharge, were analyzed in 38 spontaneous cycles. The BBT nadir was usually located at the beginning of the luteinizing hormone surge, and the first high point was 8 hours after ovulation, which was itself usually at the time when the temperature passed 37 degrees C. This temperature rise was related to the increases in plasma progesterone and 17-hydroxyprogesterone with 24 to 36 hours' delay. The BBT was found to be an unreliable technique for precise ovulation timing but would be of use if the clinical precision required for the diagnosis of ovulation were less.