The degree of nephrocalcinosis after increasing the dietary phosphorus concentration from 0.2 to 0.5 g/100 g was measured in weanling female rats of 10 inbred strains. Based on kidney calcium concentrations and histological kidney calcification scores, there were considerable strain differences in nephrocalcinogenesis; 86% of the strain variability in nephrocalcinosis was attributable to genetic factors. Two strains with the most extreme nephrocalcinogenic responses were retested and the strain difference was found to be reproducible. Mean plasma phosphorus concentrations after phosphorus feeding were lower in the sensitive strain than in the insensitive strain. The high phosphorus diet produced greater urinary phosphorus concentrations, with the increase being greater in the sensitive strain. The strain difference in the response of urinary phosphorus concentrations after raising dietary phosphorus level may determine the strain difference in phosphorus-induced nephrocalcinosis. After consuming the high phosphorus diet, RP rats housed in groups in solid-floored cages had significantly higher degrees of nephrocalcinosis than their counterparts housed individually in metabolism cages with wire-mesh bases.