A novel method for determining when cows are in estrus is disclosed comprising the step of monitoring the vaginal secretions of a cow to determine an increase in the amount of an indicator compound in said secretions. A significant increase in the amount of the indicator compound, concentrations above about 0.1 micrograms per gram of collected secretion, is indicative of estrus. The preferred indicator alcohols have ethyl ester indices of 8.1 to 8.9. In particular these alcohols are methyl-1-heptanols, such as 6-methyl-1-heptanol, or are methyl hydroxy heptenes, such as 2-methyl-7 hyroxy-.DELTA.-3-4 heptene. Alternatively, a cow may be determined to be in estrus simply to ascertaining that the concentration of methyl-1-heptanol is at least 0.1 microgram per gram of collected vaginal secretion.