To investigate how aging alters B-group vitamin metabolism, rats were fed with niacin-free 20% casein diet from 3 to 80 wk old, and the urinary excretions of the B group vitamins were periodically measured. The blood and liver B-group vitamin levels in 80-wk-old rats were also compared with those in 8-wk-old rats. The urinary excretion of thiamin, riboflavin, vitamin B₆ metabolite 4-pyridoxic acid, pantothenic acid, folic acid and biotin were not altered during 540 d. The urinary vitamin B₁₂ increased by 8-fold at 29 wk old, and further increased at 80 wk old. Conversion of nicotinamide from tryptophan gradually decreased to 60% from 29 to 48 wk old. Plasma PLP, vitamin B₁₂ and folate levels in 80-wk-old rats were lower than those in 8-wk-old rats, consistent with lower liver vitamin B₆ and folate levels in aged rats. Plasma and liver biotin levels in aged rats were higher than those in young rats. Other B-group vitamins such as vitamin B₁, vitamin B₂, niacin and pantothenic acid levels in blood and liver from aged rats were same as those from young rats. Alteration of vitamin B₆ metabolism in particular is similar to the observations in eldery humans reported previously. Our findings suggest that aged rats can be useful models to investigate aging-related B-group vitamin metabolism.