The histometric method is often used in hypertension studies in order to reconstruct, from cross-sectional profiles of vessels in the contracted state, a hypothetical relaxed state of the vessels based on the measured length of the internal elastic lamina (IEL). In this study, the accuracy of the histometric method was tested by comparing values obtained with the histometric method from contracted vessels versus values from morphometric measurements of vessels of the same calibre but maximally relaxed. Mesenteric arteries from spontaneously hypertensive and normotensive Wistar-Kyoto rats were used. We found that the lumen size of the vessels was dependent on the methods of measurement and preparation. Lumen size, determined with the histometric method from contracted vessels, tended to be underestimated mainly because the IEL became shortened during contraction of the vessels. When vessels were prepared by perfusion fixation, a high perfusion flow rate (7.6 ml/min) caused expansion of the lumen in muscular (lumen diameter 120–250 µm) and arteriolar vessels (40–100 µm), but not in the elastic arteries (400–700 µm). The cross-sectional area of the vessel wall, however, remained unchanged when the vessels were either contracted with norepinephrine or expanded with high transmural pressure, so morphometric measurements of the vessel wall and its components can be done on both contracted and relaxed vessels. Our results suggest that the use of the histometric method for the estimation of relaxed lumen size from the contracted vessels is of very limited value. We also suggest that, for an accurate comparison of vessel wall dimensions between hypertensive and ñormotensive animals, cross-sectional area should be used instead of wall thickness alone.