Investigation of the mechanisms of phenotypic realization of allelic polymorphism of the eNOS gene has shown that the level of eNOS mRNA and activity of this enzyme in platelets depends from genotype. We identified a T(-786)-->C polymorphism in the promoter region, a variable number of tandem repeats (4a/4b) in intron 4 and the G(894)-->T polymorphism in exon 7 of the eNOS gene in isolated human platelets. We measured eNOS mRNA in isolated platelets by reverse transcription-PCR and eNOS enzyme activity by fluorimetric detection system FCANOS-1 using diaminofluorescein diacetate (DAF-2A). It was shown that the level of eNOS mRNA is the lowest for the -786C/C promoter genotype. In exon 7 homozygotes (894T/T) the level of RNA is lower than in normal homozygotes (894G/G), but higher than in heterozygotes (894G/T). The eNOS activity in platelets is lower in carriers of the 786C/C promoter genotype than in normal homozygotes (2.1 times; P=0.03), and lower comparing to heterozygotes (2.9 times; P>0.05). The eNOS activity accompanying the 894T/T variant of exon 7 is also lower than in normal homozygotes (P>0.05). Regarding the polymorphism in intron 4 - the enzyme's activity is lower in carriers of the 4a/4a genotype comparing to normal homozygotes (1.7 times; P>0.05) and lower than in heterozygotes (1.9 times; P>0.05). These results allow one to conclude that the T(-786)-->C polymorphism of the eNOS gene promoter most significantly affects the gene expression and eNOS activity.