Glomerular capillary growth was studied in kidneys in five- to 540-day-old perfusion-fixed normal or sham-operated rats (C) as well as in unilaterally nephrectomized three-day-old (NN) and 120-day-old (NA) rats. The number and volume of mature glomeruli were estimated using the fractionator. The glomerular number was unaffected by neonatal or adult nephrectomy, but the number of mature glomeruli in all rats aged five days (19.1 +/- 2.0 x 10(3); +/- SD) was significantly smaller than for all the older animals (26.5 +/- 3.1 x 10(3)). The mean glomerular volume increased 59% and 20% for the NN and NA rats, respectively, versus the C rats. A capillary unit has been defined according to the number of loops in the glomerular capillary network by the use of topology. Glomerular capillary number, estimated using a physical disector, increased 53% for NN rats and 26% for NA rats. The glomerular capillary length was estimated on isotropic, uniform random sections, and increased 47% for NN rats and 12% for NA rats. The glomerular capillary surface area increased 54% for NN rats and 14% for NA rats. The diameter of the glomerular capillaries increased 8% for the mature NN versus the C rats. The rather unexpected findings are discussed and related to interesting relationships, including the law of Poiseuille and LaPlace. In conclusion, the growth of glomerular capillaries after neonatal and adult nephrectomy is performed by branching that is making new glomerular capillaries, instead of simply lengthening the existing capillaries.