A method for monitoring mammalian reproductive cycles by monitoring variations in the quantity of one or more low molecular weight volatile compound having a molecular weight of less than 50 grams per mole present in a body constituent sample is disclosed. Samples of a body constituent selected from the group of humoral fluid, breath and body cavity air are collected from a female mammal a multiple number of times during the reproductive cycle. The quantity of a low molecular weight volatile compound in each sample is measured. In the preferred embodiment, the low molecular weight volatile compound, acetaldehyde, will be measured and monitored. Variations in the quantity of the low molecular weight volatile compound appearing in each sample is monitored to determine the phase of the mammal's reproductive cycle and to predict the occurrence of ovulation.