We studied the physiological importance of elevated serum 1,25-dihydroxy-vitamin D3 [1,25(OH)2D3] in male rats. Male and female rats of equal weight consumed either a high calcium diet (1.20% calcium; HCD), a low calcium diet (0.02% calcium; LCD), or a normal calcium diet (0.60% calcium; NCD). With all three diets, serum 1,25(OH)2D3 was higher in males than females and rose in both sexes with decreasing diet calcium. There was a direct correlation between serum 1,25(OH)2D3 and proximal duodenal 45Ca uptake (r = 0.667; P less than 0.001). With NCD, we performed a balance study and found a direct correlation between serum 1,25(OH)2D3 and both 5-day fractional calcium absorption (r = 0.703; P less than 0.01) and fractional phosphorus absorption (r = 0.679; P less than 0.01). In addition, serum 1,25(OH)2D3 was directly correlated with 5-day calcium retention (r = 0.772; P less than 0.001) and phosphorus retention (r = 0.830; P less than 0.001). Males grew faster than females (F ratio = 1,194; P less than 0.001). Elevated 1,25(OH)2D3 in males appears to be of biological significance and may help satisfy a greater, growth-related, need for calcium.