A ‘normal range’ for salivary progesterone concentrations has been established using data derived from women who were menstruating regularly and in whom dating of the cycle by accepted criteria was possible. Since these values are in agreement with those in the first 9 days of conception cycles and with those in cycles in which ovulation was confirmed by ultrasonography, they provide a reliable index of progesterone output compatible with fertility. Measurement of daily salivary progesterone values in subfertile women for time spans exceeding 3 months allowed accurate assessment of base-line ovarian activity and of the response to ovulation-induction therapy. Salivary sampling, by allowing collection of frequent samples with a minimum of time, stress and inconvenience, is ideally suited to longitudinal studies of ovarian activity. This sampling regimen is also applicable to the monitoring of progesterone output throughout pregnancy.