The structural counterpart of the increased glomerular filtration found in acute diabetes mellitus and experimental diabetes has been ascribed to the increased glomerular filtration surface. Using modern design-based stereological methods and light microscopy on perfusionfixed rat kidneys the average total surface area of capillaries per glomerulus in control rats was 291 + 42 10 -3 mm 2 ( + SD) increasing to 383 + 55 10 3 mm 2 in 10-day-diabetic rats. There was a further increase to 469 + 70 10 3 mm 2 in 50-day-diabetic rats. The average total length of capillaries per glomerulus increased from 12.5 + 2.2 mm in the control rats to 16.9 + 2.4 mm in the 10-day-diabetic rats whereas the further increase to 19.4 +_ 3.0 mm in the 50-day-diabetic rats failed to reach statistical significance. The average number of topologically defined capillaries per glomerulus increased from 215 + 29 in the control rats to 260 + 45 and 316 + 29 in the 10-day-diabetic and 50-day-diabetic rats, respectively. The results showed just a slight increase in the mean length of a capillary from 58.1 + 6.2 'am in the control rats to 65.6 + 2.6 gm in the 10-day-diabetic rats after which it normalized in the 50-day-diabetic rats to 61.3 _+ 3.6 am. The geometric factor or "resistance" in Poiseuille's law did not differ between the three groups, neither did the diameter of the capillaries, nor the number of glomeruli. The implications of the formation of capillaries in renal glomeruli are discussed and it is concluded that glomerular capillaries in experimental diabetes grow mainly by the formation of new capillaries.