In contrast to other metal−dithiocarbamate [DEDTC] complexes, the copper−DEDTC complex is highly cytotoxic, inducing oxidative stress, preferentially in tumor cells. Because nitric oxide (NO) forms adducts with Cu[DEDTC]2, we investigated whether NO donors like S-nitroso-N-acetyl penicillamine (SNAP) or sodium nitroprusside (SNP), and nitrite, a NO decomposition product, modulate Cu[DEDTC]2 cytotoxicity against human tumor cells. We show that apoptosis-associated PARP cleavage and inducible nitric oxide synthase (iNOS) down-regulation induced by nanomolar Cu[DEDTC]2, are counteracted by 50 μM SNAP, SNP, or CoCl2, an inducer of hypoxia and NO signaling. Nitrite was stochiometrically effective in antagonizing Cu[DEDTC]2 cytotoxicity and inducing shifts in the absorption spectrum of the binary complex in the 280 and 450 nm regions. Subtoxic concentrations of Cu[DEDTC]2 became lethal when tumor cells were pretreated with c-PTIO, a membrane-impermeable scavenger for extracellular NO. Our results suggest that: (a) reactive oxygen species induced by Cu[DEDTC]2 are scavenged by nitrite released from NO, (b) the extent of lethality of Cu[DEDTC]2 is dependent on the reciprocal formation of an inactive ternary Cu[DEDTC]2NO copper−nitrosyl complex.