In experiments in vivo and in vitro on the mitochondria iso­lated from adult and old rat hearts, we studied the effects of a donor of hydrogen sulfide (H2S), NaHS, and H2S biosynthesis substrate, L-cysteine, on the sensitivity of the mitochondrial permeability transition pore (mPTP) opening to its natural inductor, Ca2+. We found that NaHS (10-12 to 10-4 mol/l) influ­enced mitochondrial swelling in a concentration-dependent manner. It was also demonstrated that the addition of NaHS (10-12 to 10-8 mol/l) to the calcium-free medium resulted in moderate a swelling of mitochondria from both adult and old rat hearts. At 10-10 mol/l NaHS, the maximal values of the mitochondrial swelling observed in both adult and old hearts were 11 and 15 ,%, respectively. A specific inhibitor of KAT P -channels, 5-hydroxydecanoate (5-HD; 10-4 mol/l) decreased the mitochondrial swelling in the presence of NaHS (10-10 mol/ l), which can be indicative of the contribution of these channels to the calcium-independent conductance of the mitochondrial membranes in the rat hearts. The H2S donor NaHS used in physiological concentrations (10-6, 10-5 and 5 10-5 mol/l) exer­ted the inhibiting effect on the Ca2+-induced mPTP opening in adult hearts (corresponding values of such effect were 31, 76, and 77%, respectively), while in old hearts the protector ef­fect of NaHS was observed only at its concentration of 10-5 mol/l. Therefore, the donor of H2S used in the tested concentrations (10-12 to 10-4 mol/l) exerted ambiguous effect on the mitochondrial swelling: low concentrations of NaHS (10-12 to 10-8 mol/l) increased the mitochondrial swelling, while its physiological concentrations (10-6 to 5 10-5 mol/l) exerted the protective effect on Ca2+-induced mitochondrial swelling in adult and old hearts. Pre-incubation of isolated mitochon­dria with 5-HD (10-4 mol/l) resulted in a decrease in the protec­tive effect evoked by NaHS (10-5 mol/l) on Ca2+-induced mPTP opening, which is indicative of the possible involvement of mitochondrial KAT P -channels in the H2S-dependent inhibition of mPTP formation in both adult and old rat hearts. In experi­ments in vivo, single intraperitoneal injections of both NaHS (10-4 mol/kg) and L-cysteine ((10-3 mol/kg) resulted in a de­crease in the sensitivity of mPTP to its inductor Ca2+ in adult and old rat hearts. The action of L-cysteine, as compared with that of NaHS, was more effective in prevention of Ca2+-in-duced mitochondrial swelling. We observed a rise in Ca2+ concentration by one order of magnitude, which evoked the mitochondrial swelling in adult and old hearts. In experiments in vivo in which we used a specific blocker of cystathionine-g-lyase, propargylglycine (10-4 mol/kg) that is involved in the synthesis of H2S, we observed an increase in the sensitivity of mPTP opening in old hearts because of a decrease in the thresh­old Ca2+ concentration required for mitochondrial swelling by two orders of magnitude. We demonstrate the involvement of endogenous H2S in the control of mPTP formation in adult and old hearts. Our studies are indicative of the involvement of H2S in modulation of changes in the permeability of mitochon-drial membranes, which can be an important regulatory factor in the development of cardiovascular diseases.