The aim of our work was to verify the effect of urapidil on membrane ion transport systems. This was a randomized, double-blind, cross-over study which evaluated the clinical and biochemical effects of urapidil (30 mg twice daily in comparison with placebo) in a group of 10 elderly hypertensive patients (3 male, 7 female ranging from 68 to 90 years, mean age 79.2 +/- 7.6 years). For the evaluation in fresh erythrocytes of principal ion transport systems (cotransport Na+/K+, countertransport Na+/Li+, Na+/K+ ATPase pump. intracellular Na+ and K+) we used the nystatin technique. We found that urapidil activated the red cell membrane ions cotransport system (basal values: 83.7 +/- 50.3 mumol Na+ RBC 1-1.h-1, after 1 month of urapidil therapy: 181.5 +/- 89.3 mumol Na+ RBC 1-1.h-1) (P less than 0.01), without significant changes in the other biochemical parameters evaluated. Our data suggest that one of the mechanisms of the urapidil antihypertensive effect could involve an increase in the membrane sodium cotransport system.