Our interest in the study of volatile sexual pheromones given off by cows as they enter estrus caused us to use blood head-space gas chromatography (HSCG) to monitor changes of volatile compounds during 18 complete estrous cycles of five adult cows. We observed several distinct components in the chromatographic profile, but only one component changed in concentration with the stage of the estrous cycle. Analysis of 164 blood samples revealed that the size of the peak representing this compound varied widely during the cycle and among cows. However, all cows typically showed a similar qualitative pattern of a rise and then a fall in peak area just prior to or at the time of standing estrus (the day when cows will stand still for mounting). This peak of interest eluted from the chromatographic column in less than 3 min at room temperature. Thus, we knew that the molecule was small and highly volatile. This was confirmed by preliminary mass spectral analysis, which suggested some candidate compounds. Spiking of blood samples with small amounts of these candidate compounds revealed that acetaldehyde co-eluted with our peak of interest. To collect sufficient quantity of this compound for mass spectral identification, we tested various ways to increase head space concentration. Best results occurred by adding potassium carbonate to the blood and heating it for 30 min at 90°C. This head-space gas was then derivatized by bubbling it through an acidified solution of 2,4-dinitrophenylhydrazine. Mass spectral analysis of the derivatized product confirmed that our compound of interest was acetaldehyde. We suggest that sexual hormones regulate the metabolic production of acetaldehyde. Estrus and ovulation could potentially be predicted by nondashinvasive monitoring levels of acetaldehyde in body fluids such as saliva, milk, sweat or even breath. © 1994.