Plasma progesterone levels were measured in three groups of cycles during which ultrasonic monitoring of follicular growth was performed to determine the day of follicular rupture and, thus, the mid-luteal phase when sample timing was optimal. This was in an attempt to fix accurately the lower limit of plasma progesterone in the mid-luteal phase that is associated with ovulation. Plasma progesterone levels were determined in fifteen normal cycles (group I), seventeen Clomid-treated cycles (group II) and seven conceptional cycles (group III), during which ultrasonic monitoring of follicular growth was performed. The day of sampling was also compared with the peak values of pregnanediol-3 alpha-glucuronide relative to the day of follicular rupture in corresponding cycles. This was to ascertain if sample timing was optimal. Progesterone levels in the treated cycles were generally higher than in spontaneous cycles. Results show that the mean value is statistically lower in groups I and II (t = -2.65 with 30 degrees of freedom), whereas there is no statistical difference between groups II and III or I and III. A level of 29.0 nmol/l (the lower value within 95% confidence interval in the conceptional group) is an appropriate lower index of ovulation.