We studied the relative synthetic phase (S-phase) ratio of cardiac myocytes of spontaneously hypertensive rats (SHR) utilizing monoclonal antibody to 5-bro-mo 2'-deoxyuridine (BrdU), a halogen derivative of thymidine. BrdU was administered intraperitoneally to SHR and Wistar Kyoto rats at 66.7 mg/kg/day for 3 consecutive days. The rats were sacrificed I week after the first injection of BrdU. Using anti-BrdU antibody, myocardial tissue was stained by the avidin biotinylated peroxidase complex method. We assessed the ability of the nuclei to synthesize DNA. The percentage of BrdU-labeled myocyies was higher in SHR than in control rats at 9 weeks. The hyperplastic phase of heart growth in rats was presumed to occur about 3 weeks after birth. From these results, the ability of the myocardial cell to synthesize DNA is probably enhanced under the load of high blood pressure in the early hypertrophic phase following hyperplasia.