The study was carried out to determine the proximal tubular length, surface area and length of peritubular capillaries and the nephron numbers in kidneys with chronic nephropathy and varying increase in the cortical interstitial volume. Kidneys of pigs with varying chronic obstructive nephropathy were used for the experiments. Two subgroups of ureter-obstructed kidneys were defined arbitrarily according to the volume of cortical interstitium. One subgroup (I) comprised kidneys with a volume fraction of cortical interstitium less than 30% (mean 17.2%; mean of controls 9.7%). The other subgroup (II) consisted of kidneys with severe chronic nephropathy and with a volume fraction of interstitium more than 30% (mean 44.5%). Proximal tubular length and length and surface area of peritubular capillaries were assessed by conventional morphometric techniques on 1 micron thick sections of plastic embedded material. Nephron numbers were determined by a stereological method for counting glomeruli. The results demonstrated that proximal tubular length and capillary dimensions were significantly reduced in subgroup II, whereas no significant changes were observed in subgroup I. The mean number of glomeruli was not significantly different from control values in any of the subgroups. The results are in line with observations from previous quantitative analyses of proximal tubular cross-sections indicating that proximal tubular dimensions become reduced mainly at advanced stages of chronic nephropathy. The results also indicate that shortening of individual tubules rather than loss of entire nephrons is responsible for the observed reduction in total length of proximal tubules. Finally, the present observations suggest that reduced dimensions of the cortical capillary network may have pathogenetic significance for ongoing proximal tubular atrophy in chronic renal disease.