The constitutive endothelial nitric oxide synthase (eNOS) plays a major role in circulatory homoeostasis and shows genetic polymorphism. eNOS is expressed and functional in blood cells, including erythrocytes. There is limited knowledge about the consequences of eNOS genetic variability in haemorheological parameters and erythrocyte functioning. The purpose of this study was to investigate the effects of three eNOS genetic polymorphisms, namely exonic G894T (Glu298Asp), intronic VNTR (27-bp repeat) and 5'-flanking T(-786)C polymorphisms on haemorheological variables, such as erythrocyte deformability and erythrocyte aggregation (rouleaux formation) in healthy non-smoking volunteers. Sixty subjects (19 women, 41 men) were examined for genotypes and haemorheological variables. Genotypes were determined by polymerase chain reaction and restriction analysis. Haemorheological variables were measured by means of a laser-assisted optical rotational cell analyser (LORCA). Erythrocyte aggregation was significantly decreased in individuals with 894TT genotype when compared to subjects with the (G) allele. Aggregation indices (AI) were 54.7 +/- 3.2%versus 61.0 +/- 0.9% (p = 0.026), and the half-lives (t(1/2)) for aggregation formation were 3.43 +/- 0.43 versus 2.55 +/- 0.12 sec. (p = 0.024), respectively. Similarly, VNTR-bb genotype significantly altered erythrocyte aggregability. AI values were 58.7 +/- 1.1% in subjects with VNTR-a allele versus 63.7 +/- 1.2% in subjects with bb genotype (p = 0.011); t(1/2) values were 2.86 +/- 0.16 versus 2.20 +/- 0.13 sec., respectively (p = 0.016). T(-786)C polymorphism did not change any haemorheological parameters. These findings suggest that eNOS 894TT genotype is associated with decreased erythrocyte aggregation, while VNTR-bb genotype increases aggregability in healthy human individuals. eNOS genetic variants may contribute in the pathogenesis of microvascular disorders by altering erythrocyte functions in human beings.