This study attempted to determine whether the autohemodilution that has repeatedly been shown to be induced by nitrates and other vasodilators is also produced by cilazapril. Fifteen healthy male volunteers were given 2.5 mg oral cilazapril daily for two weeks. Blood rheology was quantified by measuring blood and plasma viscosity, hematocrit count, red blood cell aggregation and deformability, and white blood cell clogging at baseline, four hours after first dose and four and 24 hours after the last dose. No acute or chronic changes were observed except for a significant increase in red blood cell deformability and a decrease in white blood cell clogging. It is concluded that hemodilution secondary to vasodilation may occur but be offset by hemoconcentration induced by aldosterone inhibition and decrease of anti-diuretic hormone levels. The changes in deformability and clogging may benefit microcirculatory function, a hypothesis that would need further investigation in controlled trials.