The time of ovulation was assessed by direct observation and microscopic examination of ovaries and oviducts from 35 mature beagle bitches killed on the first to the sixth days of estrus. These observations were related to the onset of behavioral estrus (defined as the first day of acceptance of coitus) and plasma levels of estrogens, progesterone and luteinizing hormone (LH). In most bitches ovulation was estimated to occur on the second or third day of estrus (mean = 2.9, SEM = 0.3, n = 32). A sharp peak in LH values occurred approximately at the onset of estrus, and ovulation regularly followed 2 days later (mean = 2.0, SEM = 0.1, n = 10). The small variation in the interval between the LH peak and ovulation suggests that plasma LH levels are the best available indication of ovulation time.