Plasma progesterone levels were determined in 16 Beagle bitches undergoing a normal oestrous cycle. The mean concentration rose during oestrus and a peak of 20.3 ng/ml was attained early in metoestrus. Considerable variation was noted between animals in respect of the magnitude and timing of the progesterone peak. The time of the highest progesterone values ranged from three days before the end of oestrus to Day 21 of metoestrus. The mean hormone levels remained elevated throughout metoestrus. On the basis of the time of the initial rise in progesterone levels during oestrus and the oestrogen peak values in three animals, it is suggested that ovulation occurs on the first or second day of oestrus.