The changes in glomerular permeability that occur during development were assessed in 1- and 6-wk-old canines by analyzing dextran-sieving curves obtained in six animals at each age. The fractional clearance of the smallest dextran molecules (18 A) was 0.97 +/- 0.02 (+/- SE) in both 1- and 6-wk-old animals, and it became progressively less at larger molecular sizes. The sieving curves were consistent with an isosporous model of a glomerular capillary. When axial changes in protein concentration were included in the mathematical model, the apparent pore radius was 62.7 +/- 1.7 and 61.7 +/- 1.69 A in 1- and 6-wk-old puppies, respectively (P greater than 0.7). The effects of developmental changes in hydrostatic pressure and renal blood flow were balanced by the increases in serum protein concentration and filtration fraction leaving the fractional clearances of macromolecules unchanged. In contrast, the total cross-sectional pore area per unit path length (Aw/delta x) increased during this 6-wk period by approximately 7.5-fold (from 1.39 +/- 0.2 to 10.55 +/- 3.0 10(-5) cm, P less than 0.0001), and the ultrafiltration coefficient rose from 0.012 +/- 0.002 to 0.093 +/- 0.012 ml X s-1 X mmHg-1 (P less than 0.0001). The findings reveal constancy of pore size and an increase in total pore area as a function of age. Analysis by classical pore theory yielded similar findings. We conclude that the predominant factor determining the rise in glomerular filtration rate during development is the large increment in Aw/delta x, which in turn is due to increases in the surface area and pore density of the glomerular capillaries.