Previous studies demonstrated that dietary glycine prevents elevated rates of cell proliferation following treatment with the peroxisome proliferator and liver carcinogen WY-14,643. Since increased cell replication is associated with the development of hepatic cancer caused by peroxisome proliferators, glycine may have anti-cancer properties. Therefore, experiments were designed to test the hypothesis that dietary glycine would inhibit the hepatocarcinogenic effect of WY-14,643. Male F344 rats were fed four different NIH 07-based diets: 5% glycine; 5% valine for nitrogen balance (control); 0.1% WY-14,643 + 5% valine (WY-14,643); 0.1% WY-14,643 + 5% glycine (WY-14,643 + glycine). Food consumption did not differ among the groups, but WY-14,643-fed rats weighed 10-25% less than expected based on previous studies. Serum glycine levels were elevated 4-5-fold by glycine-containing diets; however, the 10-fold increase in peroxisomal enzyme activity caused by WY-14,643 was unaffected by the addition of 5% glycine to the diet. After 22 weeks, livers from rats fed WY-14,643 had a similar incidence and multiplicity of proliferative lesions (foci and adenomas) to those fed WY-14,643 + glycine. Moreover, cell proliferation in the surrounding 'normal' parenchyma (labeling index approximately 4%) and foci (labeling index approximately 50%) did not differ between WY-14,643 and WY-14,643 + glycine-fed rats. However, after 51 weeks of dietary exposure to WY-14,643, glycine prevented formation of small (0-5 mm diameter) tumors by 23% and inhibited the development of medium size (5-10 mm) tumors by 64%. Furthermore, glycine prevented the formation of the largest tumors (>10 mm) by nearly 80%. Thus, glycine did not inhibit early foci formation; however, it significantly decreased their ability to progress to tumors. Moreover, the inhibitory effect of glycine was greater with increasing tumor size. These studies demonstrate that dietary glycine prevents the development of hepatic tumors caused by the peroxisome proliferator WY-14,643 consistent with the idea that it may be an effective chemopreventive agent.