Timing the day of ovulation in the bitch is important, especially when using frozen semen. Clinical signs (vulval oedema, aspect of vulval discharge) or clinical techniques (vaginal cytology, vaginal endoscopy) used to detect ovulation in the bitch are not accurate enough. Hormonal assays are therefore commonly used by veterinarians to assess ovulation in the bitch. LH assays are difficult to perform and expensive. The estimation of the day of the pre-ovulatory LH-peak using progesterone assays maylead to some errors, as some bitches show a premature slight rise of blood progesterone several days before ovulation. However, it has been recently demonstrated that the progesterone plasma level at the time of ovulation, whatever the breed, is fairly constant. Therefore progesterone assays appear as a fairly reliable technique to assess ovulation in the bitch. At the Alfort Veterinary College (France), we investigated the use of ovarian ultrasonography to detect ovulation in the bitch. The oestrus period was followed using ultrasound and hormonal assays in several bitches belonging to 36 different breeds. The day of ovulation was clearly detected in 91.7% of the bitches (44/48). In most bitches the ovulation process appeared to be completed within 24 hours. No significant difference in ovulation time was seen between the left and the right ovary. Compared with progesterone assays, ovulation detection using ultrasound increased the accuracy of diction of the ovulation in 10.1% of the bitches, in which the delay between ovulation detected by ultrasonography and ovulation estimated by progesterone levels was more than 24 hours. Furthermore, one daily ovarian scanning was sufficient to perform a correct time of ovulation. The ultrasound features of the canine ovaries just before ovulation, during the ovulation process, and just after ovulation, are also described in this article.