The mechanism of glomerular ultrafiltration in normal kidneys or after renal injury is reviewed. The role of increased glomerular plasma flow in mediating increases of nephron filtration rate is evidenced under experimental conditions resulting in filtration pressure disequilibrium along glomerular capillaries. The increase of nephron filtration in hypertrophied kidneys appears to be due mainly to a rise of glomerular plasma flow and, to a smaller extent, to an increase of glomerular capillary hydrostatic pressure, the ultrafiltration coefficient remaining unchanged. In contrast, in the early phases of experimentally induced nephrotoxic serum nephritis, a decrease of the ultrafiltration coefficient was observed; nephron filtration rate, however, remained within the normal range, as a consequence of a higher hydrostatic pressure in the glomerular capillaries of the nephritic kidneys.