Administration of nerve growth factor antiserum (NGFAS) to spontaneously hypertensive (SH) rats during the first week of life prevents or ameliorates the development of hypertension but does not prevent development of left ventricular hypertrophy or dysfunction. Morphometric analysis of electron micrographs prepared from subepicardial myocardial cells of animals sacrificed at age 82 days shows that NGFAS treatment significantly reduces the fall in cellular concentration of mitochondria characteristic of untreated SH controls. In both groups myofibrillar concentrations were identical and correspond to literature values for normotensive rats. The results, which are consistent with independent control mechanisms for growth of mitochondria and myofibrils, suggest that in this model (a) NGFAS affects mitochondrial concentration either via its effect on arterial blood pressure and/or via some other, unidentified mechanism, and (b) the decreased mitochondrial concentration in myocardial cells of untreated SH rats need not be causally related to the observed cardiac dysfunction. © 1978.