A quantitative study of compensatory adaptation in renal growth, function, and blood flow after progressive renal ablation was performed in the rat. Four weeks after surgery, renal mass increased 31% in control animals (Group A), 81% in nephrectomized rats with 50% ablation of total renal mass (Group B), and 168% in rats subjected to 75% ablation of total renal mass (Group C). Whole animal glomerular filtration rate (GFR) was reduced to 68% of control levels after nephrectomy (Group B), while renal blood flow (RBF) was 81% of control. Similarly, RBF was 68% of control in Group C with ablation of 75% of renal mass, while GFR was reduced to 49% of control. Since the adaptive increases in RBF exceeded those of GRF, the filtration fraction fell from 0.39 in Group A to 0.35 in Group B, and to 0.29 in Group C. Mean nephron GFR (whole animal GFR per total number of glomeruli) was 47 nl per min in Group A and increased 60% to 76 nl per min in Group B and 136% to 112 nl per min in Group C. Compensatory changes in mean nephron blood flow (RBF per total number of glomeruli) exceeded those of nephron filtration rate. Mean nephron blood flow increased from 215 nl per min in Group A to 404 nl per min in Group B (88%) and to 724 nl per min in Group C (237% over contol). These data indicate that the compensatory changes in growth, function, and blood flow after resection of renal mass correlate with the amount of tissue removed. Changes in RBF parallel but exceed those of mass and function, especially after extensive renal ablation. No apparent limitation to the compensatory response was seen under the conditions of this study.