This study was designed to assess the effects of administration of dietary soy on reproductive function and fertility of female Wistar rats. Four groups, each of 20 females, were used. Control animals were fed a standard cereal-based diet for rats. Treated animals were fed a standard diet supplemented with 0.7%, 1.2% or 2.4% of a soy extract. Treatment started at weaning and continued until day 7 post-partum (day of sacrifice). Growth depression was seen in the 2.4% soy group. Vaginal opening occurred earlier in females receiving soy supplemented feed when compared with controls. Analysis of vaginal smears revealed that all animals were cycling, although an increase in the mean duration of each cycle was seen in the 2.4% soy group. Uterine effects were observed in high-dose females and included increases in weight, oedema, endothelial hyperplasia and leucocytic infiltration. Vaginal modifications (i.e. inflammation, hyperkeratosis and dyskeratosis) and alterations in the distribution of follicular size in the ovaries were also observed among treated animals. These data suggest that long-term exposure to high doses of phytoestrogens can produce significant agonistic actions in several oestrogen-dependent tissues and parameters, even though in this model no clear influence on reproductive processes was observed.