We measured serum somatomedin-C/insulin-like growth factor I (Sm-C/IGF-I) concentrations in vitamin D-deficient (-D) rats from 3-7 weeks of age to evaluate the role of Sm-C/IGF-I in the growth effects of vitamin D. To exclude effects of alterations in food intake, feeding patterns, and plasma minerals, controls included the following groups: pair fed (PFC; n = 10), low calcium diet (LCa; n = 10), meal fed (MFC; n = 10), and ad libitum fed (AdLC; n = 5). The -D, LCa, and PFC groups had similarly depressed growth rates and Sm-C/IGF-I concentrations. The growth and Sm-C/IGF-I concentrations of the MFC group were less than those in the AdLC group, but greater than those in the -D, PFC, and LCa groups. In all groups Sm-C/IGF-I concentrations correlated well with food intake, indicating that calorie intake, and not vitamin D deficiency or hypocalcemia, was the primary factor in reducing Sm-C/IGF-I. In the final 4 days of the study, half of the -D rats were given 1,25-dihydroxyvitamin D3 (2 ng/g BW;-D/repleted), while half of the PFC rats were given excess food during the meal period (PFC/refed). The weight gain of -D/repleted rats surpassed that of the -D rats treated with vehicle (P less than 0.05); similarly, the weight gain of PFC/refed rats exceeded that of the PFC rats (P less than 0.01). In contrast, the food intake and Sm-C/IGF-I levels of PFC/refed rats were greater than those in PFC rats (P less than 0.01), while the -D/repleted rats did not have significantly altered food intake or Sm-C/IGF-I levels. We conclude 1) that alterations in Sm-C/IGF-I concentrations of young growing -D rats are not directly related to lack of vitamin D, but, rather, to poor nutrition resulting from reduced food intake; and 2) that serum Sm-C/IGF-I is not a primary mediator of growth in -D/repleted rats since improved weight gain resulting from 1,25-dihydroxyvitamin D3 treatment can occur, acutely, without changes in serum Sm-C/IGF-I concentrations.