Transrectal ovarian ultrasonography was performed daily in eight ewes during one interovulatory interval, using a 7.5 MHz, rigid, human prostate transducer, and a realtime B-mode scanner to record the numbers, diameters and position of all follicles > or = 2 mm in diameter and the corpora lutea in both ovaries. Blood samples were taken once a day and were analysed for concentrations of FSH, progesterone and oestradiol. During the interovulatory interval of 17.2 +/- 0.4 days, antral follicles (follicles > 2 mm in diameter) emerged on all days except for days 1, 5, 15, 16 and 17. A significant increase in the numbers of follicles emerging was seen on days 2 and 11. The ovulatory follicle (6.9 +/- 0.1 mm diameter) was retrospectively traced to emergence on day 11.1 +/- 0.3 and grew over a period of 4.1 +/- 0.1 days at a growth rate of 1.2 +/- 0.04 mm day-1. The largest nonovulatory follicles of the same period grew at the same rate as ovulatory follicles and regressed over a period of 2.6 +/- 0.2 days at a rate of 1.2 +/- 0.07 mm day-1. The mean diameter of the largest follicles seen on each day of the oestrous cycle was lowest on the day of ovulation (2.9 +/- 0.2 mm), increased from day 3 to day 5 (4.1 +/- 0.4 mm) and again from day 11 to the day before ovulation (6.9 +/- 0.1 mm; P < 0.05).(ABSTRACT TRUNCATED AT 250 WORDS)