Osteocalcin is an osteoblast-specific protein believed to be associated with events occurring during bone mineralization, which has been widely used clinically as an indicator of osteoblast function. Plasma osteocalcin concentrations (pOC) were studied in 94 (59 male, 35 female) healthy and 44 (21 male, 23 female) rachitic Nigerian children, all one to five years of age. The study was aimed at establishing a reference range for healthy Nigerian children determining any changes in plasma osteocalcin levels occurring in children with calcium-deficiency rickets. In the controls, pOC levels ranged from 3-89 ng/ml, with a mean value of 23 +/- 19 ng/ml. The values were higher in girls (29 +/- 21 ng/ml) than in boys (21 +/- 18 ng/ml), though not significantly. The controls had values consistent with other published pediatric ranges from Europe and North America. In the younger rachitic children (under 3 years) the mean pOC was lower than in the controls (P = 0.04) despite the much elevated plasma levels of 1,25(OH)2D. In the controls, pOC correlated with 1,25(OH)2D (r = 0.59, P = 0.003), alkaline phosphatase (r = 0. 22, P = 0.03), and inorganic phosphate (r = 0.27, P = 0.01). These correlations were lost in the rickets group. The findings in the controls confirm the known association between plasma 1,25(OH)2D and circulating osteocalcin levels, whereas the findings in the rickets group suggest that the stimulatory effects of 1,25(OH)2D on osteocalcin may depend on other permissive factors, such as normal circulating levels of calcium and phosphate.