A large number (about 4--5 nmol/mg of protein) of high-affinity (apparent dissociation constant at 37 degrees C: KD37 degrees C = 5 x 10(-8) M) calcium binding sites was characterized in synaptosomal membrane fractions enriched in plasma membranes that were isolated from rat brain. These sites were studied simultaneously in membranes from spontaneously hypertensive young rats (SHR) and their normotensive controls. No difference was observed between whole synaptosomes from these two substrains. However, plasma membrane-enriched fractions from SHR exhibited a reduced calcium binding capacity without a significant change in affinity. This decrease which averaged 15--20% was not due to any variation in the accessibility of calcium to its binding sites, as similar results were obtained in the presence of the calcium ionophore A23187. The reduction found in calcium binding is very similar to that previously described in erythrocyte membranes. It is envisaged that such an abnormality at nerve endings might play a role in the pathogenesis of genetic hypertension.