Cows' milk contains considerable quantities of estrogens, mainly in the form of estrone sulfate (ES). To determine whether the commercial milk has any biologically significant hormonal effects, 2 series of uterotrophic tests were performed, 1 with young ovariectomized rats and the other with sexually immature rats. Thirty-six rats were used for each test. They were divided into 3 groups of 12 animals each, and were kept for 7 days on powdered chow with 1 of 3 drinking solutions: low-fat milk (LFM), artificial milk (AM, negative control), or AM containing ES at 100 ng/ml (positive control). At autopsy, both the wet and blotted uterine weights were measured. The cell heights of uterine epithelia in ovariectomized rats were also determined. The significance of differences among groups was tested by Dunnett's multiple comparisons test. In each test, the weights of the uteri in the LFM group were significantly greater than those of the respective weights in the AM group (p< 0.01). Furthermore, in ovariectomized rats, the uterine epithelial-cell height in the LFM group was significantly greater than that observed in the AM group (p< 0.01). The uterotrophic effect of 100 ng/ml ES solution was greater than that of LFM in immature rats (p< 0.01), whereas the effect of the solution was almost comparable to that of LFM in young ovariectomized rats (p> 0.05). In conclusion, commercially available milk has uterotrophic effects in both young ovariectomized rats and sexually immature rats.