We examined the effect of vitamin K and vitamin D supplementation on calcium balance in young rats fed a normal or low calcium diet. Eighty female Sprague-Dawley rats, 6 wk of age, were randomized by the stratified weight method into eight groups with 10 rats in each group: 0.5% (normal) or 0.1% (low) calcium diet, 0.5% or 0.1% calcium diet+vitamin K (vitamin K2, menatetrenone, 30 mg/100 g, food intake), 0.5% or 0.1% calcium diet+vitamin D (25 microg/100 g, food intake), and 0.5% or 0.1% calcium diet+vitamin K+vitamin D. The duration of the study was 10 wk. Vitamin K supplementation promoted the reduction in urinary calcium excretion and retarded the abnormal elevation of serum PTH level in rats fed a low calcium diet, and stimulated intestinal calcium absorption in rats fed a normal calcium diet. Vitamin D supplementation stimulated intestinal calcium absorption with prevention of the abnormal elevation of serum PTH levels and prevented hypocalcemia in rats fed a low calcium diet, and stimulated intestinal calcium absorption in rats fed a normal calcium diet. The stimulation of intestinal calcium absorption was associated with increased serum 1,25-dihydroxyvitamin D levels. An additive effect of vitamin K and vitamin D on intestinal calcium absorption was found only in rats fed a normal calcium diet. This study shows the differential effects of vitamin K and vitamin D supplementation on calcium balance in young rats fed a normal or low calcium diet.