Using stereological techniques capillaries, interstitium and myocardial fibers were analyzed in perfusion-fixed hearts of subtotally nephrectomized male Sprague-Dawley rats with uremia of 14 months duration (or their sham-operated controls). Uremic rats had higher systolic blood pressure (140 +/- 20.3 mm Hg vs. 119 +/- 6.61 mm Hg) and left ventricular weight/body weight ratio (3.37 +/- 0.09 mg/kg vs. 2.01 +/- 0.12 mg/kg) than controls, and had slight anemia (Hct 35.0 +/- 3.16% vs. 40.4 +/- 3.3%). Length density (Lv) of capillaries, that is, capillary length per unit myocardial volume, was significantly (P < 0.001) decreased in uremia (2485 +/- 264 mm/mm3 vs. 3329 +/- 194 mm/mm3) versus controls. In parallel, surface density and volume density of the capillary lumina were also reduced (7.95 +/- 1.69 cm3/cm3 vs. 11.4 +/- 1.8 cm3/cm3) in the uremic rats. We conclude that in experimental uremia, cardiac hypertrophy is not accompanied by a commensurate increase in capillaries.