Individual antral follicles from 11 horse mares were studied at three stages of the oestrous cycle to determine the characteristics of the presumptive ovulatory follicle. Mares were ovariectomized (ovex) during the late luteal phase on Day 14 after ovulation (Group 1) and on the 1st (Group 2) or 4th (Group 3) day of oestrus. Every follicle greater than 5 mm in diameter was dissected from each ovary; follicles greater than or equal to 15 mm in diameter were analysed separately while others were pooled by size for subsequent analyses. The presumptive ovulatory follicle possessed the following characteristics: in Groups 2 and 3 they were the largest in size; in all groups they contained the largest amount of protein in the granulosa cell component; they were the most vascular; they contained the highest follicular fluid oestradiol concentration; and they possessed the highest granulosa cell LH/hCG receptor content. In 9 mares, there was one such follicle present. In 2 mares, there were 2 such follicles present and one of these mares had a history of repeated double ovulations. The presumptive ovulatory follicle of Group 1 mares contained more thecal LH receptor than did non-ovulatory follicles. There were no differences in granulosa FSH receptor content amongst follicles, although smaller follicles tended to have increased FSH receptor content. Follicular fluid FSH and LH reflected peripheral hormone concentrations with follicular fluid LH being highest in presumptive ovulatory follicles of Group 3 mares. Follicular fluid progesterone concentrations tended to be higher in presumptive ovulatory follicles when compared with small follicles. Within a class of follicle, follicular fluid androstenedione and testosterone concentrations were lower in follicles of Group 1 mares than in follicles from Group 2 and 3 mares, reflecting endogenous stimulation of androgen biosynthesis by LH. Elevated follicular-fluid oestradiol occurred only in the presumptive ovulatory follicle with concentrations at least 30-50-fold higher than in non-ovulatory follicles. These results demonstrate that the presumptive ovulatory follicle in horse mares has been selected by at least Day 14 after ovulation and can be identified at this time by its various biochemical characteristics. Only after the first day of oestrus were physical characteristics such as size accurately able to predict ovulatory follicle status. These results clearly demonstrate the relationship between LH stimulation of androgen biosynthesis in vivo regardless of follicle size or status (ovulatory or non-ovulatory).(ABSTRACT TRUNCATED AT 400 WORDS)