Serum 25-hydroxyvitamin D (25[OH]D), an index of vitamin D nutrition, and the calcium-regulating hormone 1,25-dihydroxyvitamin D (1,25[OH]2D) were measured in rhesus macaques of various ages. Both metabolites were much higher in monkeys than in man. For 25(OH)D there was no sex difference, and this metabolite increased with age (P greater than 0.05). The 1,25(OH)2D fell with age for males (P less than 0.02), but not for females. Pregnant or lactating females had significantly elevated 1,25(OH)2D levels (P less than 0.025).