In aortic smooth muscle cells from 12 spontaneously hypertensive rats (SHR) of the Münster strain and 11 normotensive Wistar-Kyoto rats (WKY), the intracellular Na+ content was measured by electron-probe microanalysis. Measurements were performed in aortic cryosections 3 microns thick; the Na+ content was 12.5 +/- 2.4 g/kg dry weight in SHR versus 6.96 +/- 1.1 g/kg dry weight in WKY (P less than 0.01). Thus, aortic smooth muscle cells from SHR are characterized by a markedly elevated intracellular Na+ content compared with normotensive cells. This may either be due to genetically determined disturbances in transmembrane Na+ transport or to a circulating factor affecting Na+ transport. Cellular Na+ handling may be disturbed in SHR aortic smooth muscle as it is in hypertensive blood cells.