SUMMARY1. Single-nephron and whole-kidney function were studied in female rats with corticomedullary nephrocalcinosis, and in animals where the lesion had been prevented either by a dietary magnesium supplement or by using a diet with a calcium: phosphorus ratio in excess of 1.2. At the single-nephron level, rats with nephrocalcinosis had prolonged tubular fluid transit times. Proximal transit time was 19-42 + 1-98 (mean+ S.E. of mean) vs. 1 1-58 + 0-19 s for controls; distal transit time was 62-64 + 9-16 vs. 31-50 + 1-03 s for controls.3. Although single-nephron function is altered in nephrocalcinosis, data obtained from rats in metabolism cages indicate that whole-kidney function is largely unaffected by the lesion.