Experiments were conducted to examine the effects of methionine supplementation on histidine metabolism in rats. All animals were fed 10% casein diets with a methionine content of either 0.6 or 1.1%. Experiments in which the animals were fed their diets containing an additional 1% histidine ad libitum for at least 10 days revealed that methionine-supplemented animals had a 49% reduction of plasma histidine and an 80% reduction in urinary excretion of formiminoglutamic acid (FIGLU) on day 10. This effect was not observed on day 5. In subsequent experiments rats were fed the control or test diet ad libitum prior to receiving their diets, containing a histidine load, by force-feeding. When a 100-mg histidine load was given on day 5, 24-hour urinary FIGLU excretion was 83% lower in methionine-supplemented animals. When rats were force-fed a 75-mg [ring-2-14C]histidine load on day 10, those receiving supplemental methionine oxidized 21% more of the histidine label to 14CO2 and excreted 61% less of the dose as urinary FIGLU in 24 hours. The activities of histidase and urocanase were unaffected by the methionine supplement. The results suggest that dietary methionine supplementation enhances the in vivo catabolism of histidine by stimulating one-carbon metabolism. Delivery of the methionine supplement by ad libitum feeding requires at least 5 days for this effect to be achieved.