The serum concentrations of the vitamin D metabolites 25-hydroxyvitamin D2 (25OHD2), 25-hydroxyvitamin D3 (25OHD3), and 1,25-dihydroxyvitamin D (calcitriol) have been measured in normal subjects whose ages varied from 18 months to 35 yr. Samples were obtained in all months of the year in order to assess the effects of season on serum concentration. During the months of April to September, 25OHD3 levels are higher than in the winter months. No seasonal variation in the 25OHD2 or calcitriol serum concentration was observed. Age-related differences in 25OHD2 and D3 concentrations did not exist. The levels of calcitriol are higher in adolescence and increase from 35 +/- 19 pg/ml (SD) at 1.5-10 yr of age to 54 +/- 21 pg/ml at 10-20 yr of age. In young adults, the levels fall again to 28 +/- 16 pg/ml. Accordingly, despite a seasonal variation in the precursor of calcitriol, the levels of this most active metabolite of vitamin D do not change in relation to sunlight exposure. This lack of seasonal variation is further evidence of the tight feedback regulation of calcitriol.