Mildronate [3-(2,2,2-trimethylhydrazine) propionate (THP)] is an antiischemic drug acting mainly via inhibition of fatty acid β-oxidation. Some effects of the drug cannot be explained by the latter mechanism. We tested the eventual nitric oxide (NO) dependence of the mildronate action. Mildronate, γ-butyrobetaine (GBB) and GBB methyl ester induced transient increases in nitric oxide (NO) concentrations in rat blood and myocardium. In vitro, these compounds neither modified the activities of purified neuronal and endothelial recombinant nitric oxide synthases (NOSs) nor were able to interact with their active site. GBB induced vasodilatation at high concentrations only (EC50=5×10−5 M) while mildronate alone displayed no vasodilating effect although it enhanced the GBB vasodilating activity. GBB methyl and ethyl esters were found more potent vasodilators (EC50=2.5×10−6 M). Pretreatment of aortic rings with NOS inhibitor Nω-nitro-l-arginine methyl ester (l-NAME) abolished vasodilating effects of the compounds. A hypothesis explaining NO and endothelium-dependent effects of mildronate and its analogues is proposed.