Many mares enter a period of spring transition between winter anoestrus and cyclicity when follicles may reach preovulatory size, yet fail to ovulate. The aim of the present study was to determine the morphological, functional and molecular characteristics of large dominant transitional follicles. Follicle growth was monitored regularly from February through July in 25 pony mares. In some of the mares, one of the ovaries was removed on the day after an anovulatory follicle reached 30 mm, and the contralateral ovary was removed during cyclicity on the day after the preovulatory follicle reached 30 mm. Samples of the large follicle were processed for immunohistochemistry and in situ hybridisation, and follicular fluid was frozen. Hormonal control of growth of the large transitional follicles was similar to that seen during cyclicity. However concentrations of oestradiol were significantly lower than during growth of the preovulatory follicle and LH remained low until ovulation. During growth of anovulatory follicles, uterine oedema was sometimes present along with low, but significant, elevations in oestradiol. Transitional follicles had poorly developed theca that received a scant blood supply and there was little VEGF, the main ovarian angiogenic factor, in these follicles. Transitional follicles contained only low concentrations of oestradiol and progesterone and there was low expression of mRNA encoding StAR, the steroidogenic enzymes, and LHr in the follicle walls. Our results show that the poor steroidogenic capacity of transitional follicles is directly related to low gene expression of the steroidogenic enzymes, possibly caused by the low levels of LHr and IGF mRNAs. The scant blood supply and poor development of the theca would also contribute to the steroidogenic incompetence of these follicles. Expression of IGF-I and-II mRNAs were lower in transitional anovulatory follicles than in preovulatory follicles and intrafollicular concentrations of IGFBP-2 were significantly higher in transitional than in preovulatory follicles. These results suggest that the bioavailability of intrafollicular IGF is enhanced in large preovulatory follicles during the breeding season and suggests that intrafollicular IGF bioavailability must exceed a threshold level before an ovulation can occur.