This study investigated the duration of the interval between the onset of vulval bleeding at pro-oestrus and ovulation estimated from the plasma progesterone concentration in a large number of beagle bitches. The influence and association of individual variation, ageing and duration of the oestrous cycle were also investigated. The mean time of ovulation after the onset of vulval bleeding was 11.1 ± 0.2 days, but it widely ranged from 3 to 31 days. This timing was not influenced by age or duration of the oestrous cycle, and within-individual variation was small. As there has been no previous report in which the ovulation day was investigated by the age, these data may be very valuable.