A radioimmunological method of salivary LH determination has been developed as a new non invasive approach to hormonal ovulation detection. In this study salivary LH patterns have been compared to serum LH peak and daily sonographic assessment of follicle maturation in 15 spontaneous cycles of 9 women experienced in self-observation of their cycles (NFP). The day where the mature follicle was no longer visible sonographically was labelled day 0. Serum LH peaks occurred in 11 out of 15 cycles on day -1, two times on days -2 and 0 and preceded salivary LH peaks in 8 out of 15 cycles. The latter coincided in 7 cycles with day 0 in two cycles each with days -1 and +1 and in 4 cycles with day -2. Thus 13/15 serum- and 11/15 salivary-LH peaks occurred within +/- one day of the disappearance of the mature follicle. A time lag of up to six hours between the sampling of saliva and serum might explain the difference in the respective peak days, however, more studies into the kinetics of LH transport and its circadian rhythmicity seem necessary. Nevertheless, in principle also the salivary LH peak is considered a suitable indicator of ovulation.