The effect of protein deficiency on plasma 25-hydroxyvitamin and 24,25-dihydroxyvitamin D concentrations was determined in weanling rats. The concentrations of these metabolites did not differ in rats fed diet providing either 0.5 or 21% casein for 4 weeks postweaning. Also, no change was observed in plasma levels of these metabolites in rats fed the 21% casein diet in amounts restricted to those consumed by the 0.5% casein animals for the same period of time. The results indicate that the synthesis and transport of 25-hydroxyvitamin and 24,25-dihydroxyvitamin D are not significantly affected by either a protein deficiency or a total food striction during the early postweaning period. This finding may be relevant to the vitamin D status of children with kwashiorkor.