Ewes actively immunized against alpha N, the N-terminal peptide of inhibin alpha 43 precursor, have lowered fertility associated with ovulation failure, restricted tissue remodelling and reduced matrix metalloproteinase-2 activity in the follicular fluid at the time of expected ovulation. This could be due to altered ratios of matrix metalloproteinase-2 and tissue inhibitor of matrix metalloproteinase (TIMP-1), or to the onset of atresia in antral follicles destined to ovulate. The objectives of the present study were to investigate the effects of immunization against alpha N on the localization of TIMP-1 in ovine follicles, and on follicular growth and atresia in the follicular phase. Ewes were either immunized against alpha N or remained as controls and the ovaries were removed before (0, n = 4) and at 12 h (n = 4) and 24 h (n = 4) after hCG administration in a synchronized follicular phase, 48 h after removal of intravaginal pessaries. Observations were made on a single section taken through the largest follicle present in the ovaries of each ewe. There were no healthy antral follicles > 1 mm in immunized ovaries (0/29) compared with controls (16/31) (P < 0.001), whereas the proportion of healthy antral follicles < 1 mm was the same in each group (9/19 versus 5/12). TIMP-1 immunoactivity was localized in large luteal cells, smooth muscle and endothelial cells, and in all antral follicles, including oocytes. At the time of hCG administration, no TIMP-1 immunoreactivity was detected in the apical region of the follicular wall of large follicles (> 6 mm) compared with the rest of the follicle wall, but staining appeared in the apical granulosa layer 24 h later. In newly formed corpora lutea, TIMP-1 expression was found along the invaginating vascular layer. There was no effect of immunization on the patterns of TIMP-1 immunoreactivity, suggesting that changes in TIMP-1 are not involved in the effects of alpha N. These data are consistent with a paracrine role for alpha N in the selection and atresia of antral follicles, and for TIMP-1 in tissue reorganization and steroidogenesis at the time of ovulation.