The development of the permanent kidney, or metanephros, is a complex process. In the present study, stereological methods were used at the light microscopic level to estimate the absolute volumes and volume densities of seven compartments in the developing rat metanephros, from embryonic day 14 (E14) to E21. Metanephroi from time-mated Sprague-Dawley rats were embedded whole in glycolmethacrylate, exhaustively sectioned at 2 microm and stained with PAS. The left metanephros from three embryos from each of three mothers were analysed at each of the ages (a total of 72 metanephroi). Relative volumes were multiplied by total metanephric volume to obtain absolute volumes. Total metanephric volume increased approximately 300-fold during the 7-day period studied. At E14, 92% of the metanephros was composed of undifferentiated mesenchyme, whilst the ureteric epithelium made up approximately 5% of the volume. By E21 the undifferentiated mesenchyme comprised 47% of the kidney, while the ureteric epithelium comprised 9% and the various components of developing nephrons (epithelial vesicles, comma-shaped bodies, S-shaped bodies, glomeruli, tubules) comprised 43%. Equations with prediction intervals describing the growth of the whole kidney as well as the absolute and relative growth of the seven kidney compartments were generated. These data provide a baseline for future studies on the roles of specific molecules in renal development.