The membrane structure of erythrocytes of rats with different forms of arterial hypertension was studied by means of 2 hydrophobic fluorescent probes (diphenylhexatriene and pyrene). Microviscosity of hydrophobic areas of erythrocyte membrane of spontaneously hypertensive rats was increased compared with that of membranes from normotensive control rats. No alterations of membrane structure of erythrocytes of deoxycorticosterone-treated rats and renal hypertensive rats were found.