Soybean isoflavones are structurally similar to mammalian estrogens and therefore may act as estrogen agonists or antagonists. Research has not been determined if these soybean isoflavones have any negative effects on reproductive parameters in male livestock. The objective of this study was to evaluate the effects of soybean isoflavones on male reproduction using Chinese mini-pig boars as a model. Fifty Xiang boars were randomly divided into five groups and fed diets containing 0, 125, 250, or 500 ppm soybean isoflavones or 0.5 ppm diethylstilbestrol for 60 days. The results of this study indicate that consumption of soy isoflavones at dietary levels up to 250 ppm did not adversely affect reproductive parameters in Chinese mini-pig boars whereas higher levels of soy isoflavones may adversely affect male reproduction. Higher levels of soy isoflavones produced some tissue changes and chemical responses in this test pig model.