The possibility that erythrocytes may function as a reservoir for noradrenaline and adrenaline and as a modulator of circulating catecholamine concentrations had been suggested. The aim of this work was to study the adrenaline effect on erythrocyte membrane fluidity, acetylcholinesterase (AChE) enzyme activity, P(50) and erythrocyte deformability and also to verify if the role of adrenaline on erythrocyte properties is sex-dependent. Blood samples from 42 healthy donors were obtained, and its aliquots incubated 30 min without (control) and with 10(-5) M concentrations of adrenaline alone (A(1)) and adrenaline with an alpha and an beta-blocker (A(2)). Results demonstrate that initial AChE values in female are higher (p<or=0.01) than male values. In female, adrenaline decreases AChE activity either when alpha and beta-adrenergic receptors are blocked (p<or=0.01) or when they are not. In male, adrenaline increases AChE activity when none of adrenergic receptors are blocked. Control values of male and female erythrocyte membrane fluidity are very similar but behaviour became differently (p<or=0.05) when adrenaline is present because it decreases male and increases female values. Gender differences in erythrocyte deformability are verified at high shear stress values (p<or=0.02). In female we have also registered the existence of an inverse significant correlation (r=-0.62) between membrane rigidity and AChE activity in A(2) values. Adrenaline increases p(50) values (p<or=0.03) in both sexes. Peripheral blood film has shown echinocytes when adrenaline 10(-5) M is present. We conclude that in this in vitro study sex-related differences in erythrocyte acetylcholinesterase enzyme activity, membrane fluidity and erythrocyte deformability under adrenaline influence were found.