hydrogen sulfide (h2 s) has emerged as a novel and important gasotransmitter for the cardiovascular system, where it is generated mainly by cystathionine γ-lyase (cse). abnormal metabolism and functions of the h2 s/cse pathway have been linked to cardiovascular diseases including hypertension and atherosclerosis. hypoxic stress is a hallmark of these pathophysiological processes. here we characterized the response of vascular smooth muscle cells (smcs) derived from cse-knockout (ko) mice to hypoxic stress. under basal conditions we found that ko cells exhibited increased metabolic activity (+27 %), ros levels (+127 %) and mitochondrial membrane potential (Ψ m ) (+62 %) than their wild-type (wt) counterparts. these data suggest an impaired ability of ko cells to regulate redox levels and/or increased by-production of ros resulting from deficient h2 s-mediated regulation of mitochondrial activity in the ko cells. hypoxic insult (12 h, 1% o2 ) caused a decrease (-61 %) in ko cell metabolism and a dramatic increase in apoptosis of ko versus wt cells (+85 % versus +23 %), indicating susceptibility of ko smcs to hypoxic stress. we also found hypoxia-induced increases in the activity of antioxidant enzyme, superoxide dismutase, in both wt and ko cells, but markedly increased (+75 %) ros levels in hypoxic ko cells, revealing a profound redox imbalance therein. additionally, we noted: (i) a large Ψ m increase (+118 %) in hypoxic ko versus wt cells; (ii) that ko smcs featured altered inflammatory mediator expression both under basal and hypoxic conditions; (iii) blunted hypoxia inducible factor-α expression in the cse-deficient cells. taken together, these data demonstrate that h 2 s/cse pathway is essential for smc survival under hypoxic conditions, and suggest that endogenous h2 s modulates redox and inflammatory status and mitochondrial activity, deficiency of which may cause the observed vulnerability of ko cells to hypoxia. in another series of experiments, we examined the effect of the hypoxia mimetic, cobalt chloride (cocl2 ), as well as hypoxia-reoxygenation (h-r) stress, on hl-1 murine cardiomyocytes. we demonstrated that both cocl 2 and h-r caused decreased cardiomyocyte viability (-28 % and -37 %, respectively), and that cse expression was concomitantly increased (+79 % and +94 %, respectively). d-propargylglycine-mediated inhibition of cse was found to exacerbate cocl2 -induced decreases in cardiomyocyte viability. taken together, these data indicate that h2 s/cse pathway is upregulated in response to both simulated hypoxia and h-r stress, and that endogenous h2 s/cse pathway contributes to cardioprotection against cocl2 -induced stress. these findings suggest an important role for endogenous h2 s/cse pathway in cytoprotection against hypoxia in murine smcs, as well as against simulated hypoxia and h-r stress in hl-1 murine cardiomyocytes. moreover, they are consistent with the concept of meaningful involvement of h2 s/cse pathway in the hypoxia stress response and the recent suggestion of h2 s/hif-1 interaction.