Ventricular performance and myocardial hypertrophy were studied in maturing spontaneously hypertensive (SH) and normotensive Kyoto-Wistar (WKY) rats from 4 through 24 weeks of age. The development of hypertension in the SH rat was prevented by treatment of the newborn rat with nerve growth factor antiserum (NGFAS). NGFAS treatment had no effect on the blood pressure of WKY rats. The hearts of 1-day- and 2-week-old SH and WKY rats were also examined for evidence of hypertrophy. Cardiac indices of SH rats, regardless of NGFAS treatment, were similar to those of WKY rats until 12 weeks of age at which time they became significantly lower. Stroke indices of both SH groups were lower than those of the WKY groups at all ages studied. Heart rates, however, were significantly higher in the SH rats during the early hypertensive phase but later declined to control levels. NGFAS treatment, despite its effects on hypertension, did not prevent the development of myocardial hypertrophy. Left ventricular hypertrophy was found in SH rats from age 1 day to 24 weeks regardless of NGFAS treatment. In SH rats 1 day to 8 weeks of age right ventricualr hypertrophy was also noted. Thus myocardial hypertrophy is present in SH rats before or very early in the course of the development of hypertension and when hypertension is prevented by peripheral immunosympathectomy. These data support our earlier conclusion that the development of myocardial hypertrophy is independent of elevated blood pressure and suggest that myocardial hypertrophy may be a manifestation of an underlying myocardial abnormality which could account for the hemodynamic alterations found in SH rats. © 1978.