Regional differences in cell size in the hearts of rats with and without cardiac hypertrophy were studied using isolated muscle cells. Isolated cardiac myocytes were prepared from left ventricular free wall inner and outer halves and the right ventricle of six male 12-week-old spontaneously hypertensive (SHR), Wistar-Kyoto (WKY) and Fischer-344 rats. In SHR, blood pressure was increased to 188 ± 4 (SEM) mm Hg versus 143 ± 2 and 133 ± 10 for WKY and Fischer rats, respectively (p < 0.001). Total heart weight was increased to 1103 ± 29 mg in SHR compared to 824 ± 21 in WKY and 951 ± 23 in Fischer rats (p < 0.001). Isolated cardiac myocytes were prepared by perfusion of isolated hearts with Ca++free Hanks’ solution containing EGTA followed by collagenase-containing media. Mean length, width and volume of 150 cells stained with hematoxylin and eosin from each site were measured with a sonic digitizer. Two nuclei were present in 85 to 87% of isolated cells from all strains and regions. There was no difference among strains in right ventricular cell length, width, or volume, nor between left ventricular inner and outer halves within each strain. Left ventricular cells were larger than right ventricular cells (p < 0.05) in all strains. Left ventricular cells of SHR were larger than left ventricular cells of WKY or Fischer rats in proportion to the increase in total heart weight, indicating that cardiac enlargement in SHR is due to increased cell size rather than increased cell number. © 1979 American Heart Association, Inc.