Several operationally defined adenosine triphosphatase (ATPase) activities were determined in vitro in red blood cell lysates of normotensive or hypertensive humans: Mg2+-ATPase, Na+, K+-ATPase, and Ca2+ pump ATPase, the latter in the calmodulin-activated and basal states. Basal Ca2+pump ATPase was defined as the Ca1+-activated ATPase resistant tol(HM trifluoperazine. Subjects were part of a double-blind study in which treatment was divided into several phases: baseline (4 weeks), placebo or calcium (1 g elemental calcium/day, 8 weeks), placebo washout (4 weeks), placebo or calcium (1 g elemental calcium/day, 8 weeks). Irrespective of the phase of treatment, the basal Ca3+pump ATPase activity in red blood cell lysates of 36 hypertensive subjects was significantly less than that in lysates from 18 normotensive subjects. Other ATPase activities did not differ significantly, although all ATPases tended to be decreased in hypertension. The data are consistent with previous reports of altered membrane Ca3+binding and transport in hypertension, but the precise changes are not elucidated.