We present a new perfusion technique that allows arteries down to the level of capillaries to be fixed while relaxed and under a known intravascular pressure. Through a catheter inserted into the right renal artery of 12-week-old male spontaneously hypertensive rats (n = 9) and control Wistar-Kyoto rats (n = 11), the kidney vessels were rinsed with human plasma, relaxed by papaverine, and perfused with a casting resin containing microspheres. The microspheres (12 microns) became trapped in the glomeruli of the kidney and, together with a closing of the venous outflow, they caused the flow through the kidney to stop, so that the intravascular pressure was raised to the level of the input perfusion pressure (100 mm Hg). The resin material was allowed to harden, and the kidney was immersion-fixed and prepared for histomorphometrical investigations. This technique made it possible to measure both the structurally determined lumen diameter and the corresponding media thickness under clearly defined conditions. The lumen diameter of afferent arterioles close to the glomeruli showed a 17% reduction in spontaneously hypertensive rats (15.4 +/- 0.6 microns; mean +/- SEM) compared with Wistar-Kyoto rat arterioles (18.5 +/- 0.3 microns, p < 0.001). However, this was not due to media hypertrophy, because media cross-sectional area was smaller (p < 0.001) in spontaneously hypertensive rats (210 +/- 6 microns 2) compared with Wistar-Kyoto rats (274 +/- 16 microns 2). We conclude that the lumen reduction in renal afferent arterioles in spontaneously hypertensive rats is not the result of an encroachment on the lumen by a hypertrophic media.