Vasorelaxant activity of new stable powder preparations of dinitrosyl iron complexes (DNIC) with thiol-containing ligands was investigated on rat abdominal aorta rings. The preparations preserve their physicochemical characteristics (EPR and optical absorption) if stored for a long time in dry air (at least half-year). Three preparations of DNIC were tested: diamagnetic dimeric DNIC with glutathione (DNIC-GS 1:2) or cysteine (DNIC-cys 1:2) and paramagnetic monomeric DNIC with cysteine (DNIC-cys 1:20). Being dissolved in physiological solution the preparations induced relaxation of vessel similarly to that by earlier described non-stable DNICs which should be stored in liquid nitrogen. The amplitudes and kinetic characteristics of the relaxation were dependent on the incorporated thiolate ligands. Rapid transient relaxation followed by significant tone recovery to stationary level (plateau) was observed for DNIC-cys 1:2. DNIC-cys 1:20 also induced initial rapid relaxation followed by incomplete tone recovery. DNIC-GS 1:2 induced slow developing and long lasting relaxation. NO scavenger, hydroxocobalamin (2x10(-5)M) eliminated the rapid transitory relaxation induced by DNIC-cys 1:20 and did not influence significantly on the plateau level. SOD increased duration of the DNIC-cys 1:2 and DNIC-cys 1:20 induced relaxation. The addition of 5x10(-5)M DNIC-cys 1:2 or DNIC-cys 1:20 induced long lasting vasorelaxation within 20min and more. However the EPR measurements demonstrated full rapid disappearance (within 1-2min) of both type of DNIC-cys in Krebs medium bubbled with carbogen gas. This was not the case for DNIC-GS 1:2. We suggested that the long lasting vasorelaxation observed during the addition of DNICs-cys was induced by S-nitrosocysteine derived from DNICs-cys and stabilized by EDTA in Krebs medium. The suggestion is in line with the fact that strong ferrous chelator bathophenantroline disulfonate (BPDS) which is capable of rapid degradation of DNICs did not abrogate the vasorelaxtion induced by DNIC addition.