The ovaries of 17 Friesian cows aged two to seven years were examined daily by transrectal ultrasound from one week after calving. The position and size of all follicles more than 2 mm in diameter were recorded. A large (more than 9 mm in diameter) follicle was present on at least one ovary by six to 17 days after calving, and in two of the cows it ovulated; the remaining cows had from two to nine large follicles before they ovulated. The mean (SEM) interval from calving to the first ovulation was 43.4 (5.3) (range 13 to 93) days, after 4.2 (0.6) waves of follicles.