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Computer operator-interactive planimetric methods that have been employed to measure volume of a selected tissue in serial computed tomographic sections are time consuming, and the accuracy of the measurement is limited by the operator's skill in outlining the tissue. We introduce an alternate method that is 5 to 6 times faster than planimetry and is independent of the operator's outlining dexterity. This method involves the random marking of voxels in a three-dimensional array of known size and subsequent identification of the proportion of those voxels that are within the tissue of interest. Tissue of the kidney and left ventricular myocardium were imaged in situ with a high speed, volume scanning, computerized x-ray tomographic imaging system. Volumetric measurements made by the random marking method correlatedhighly (r=0.99) with measurements made by planimetry and with postmortem weight. These data indicate that the random marking method provides a rapid and accurate means of estimating tissue volume in serial tomographic sections.