In the small intestine, microvillus (MV) height is usually thought to be directly related to apical absorptive surface area. To evaluate whether MV height or other MV structural features were real indicators of absorptive surface area, we evaluated, by computerized morphometric analysis, the correlation between apical absorptive surface area/cell (AASA) and key MV structural features. These included, MV height, MV No./cell, and the non-MV amplified flat cell surface of rat jejunal (J) and ileal (I) absorptive epithelial cells. We studied 240 cells/region from defined mid and tip villus loci. AASA was defined as flat surface/cell x No. MV/cell x mean MV surface, MV No./cell as MV/u2 x flat surface/cell and MV surface as u2/MV. In J, AASA increases from 679 ± 15 u2/cell to 1085 ± 30 u2/cell (p<.01), between villus mid and tip, and similarly in I. There was a strong positive correlation between AASA and MV No./cell in J and I (J, r=+.870, p<.01; I, r= +.889, p<.01), but no correlation between MV height and cell surface area (J, r=+.290). And, MV height was indistinguishable at villus mid and tip loci (mid; 1.4 ±.3, tip; 1.6 ±.3 u), despite the increase in AASA. Our data indicate that AASA is dependent on MV No./cell and that MV height is not a measure of AASA, except in cases of severe MV damage. These observations hint that apparently normal sites, in patchy intestinal lesions, may also be compromised.