Summary. A marked difference in both the function and composition of individual ovarian follicles was noted in Booroola × Romney ewes (6–7 years of age) which had previously been segregated on at least one ovulation rate record of 3–4 (F+ ewes, N = 21) or 6 (s.e.m.) granulosa cells, whereas in ++ ewes (N = 3), such follicles were 7·3 ± 0·3 mm in diameter and contained 6·5 ± 0·8 × 106 cells. During a prostaglandin (PG)-induced follicular phase, the secretion rate of oestradiol from ovaries containing 3 presumptive preovulatory follicles in F + ewes was similar to that from ovaries with only one such follicle in ++ ewes. We suggest that the putative 'gene effect' in F+ ewes is manifested during early follicular development and that it may be mediated via an enhanced sensitivity of granulosa cells to pituitary hormones. As a consequence, the development of 3 preovulatory follicles in F+ ewes may be necessary to provide a cell mass capable of producing the same quantity of oestradiol as that from one preovulatory follicle in ++ ewes.