In this study, we further examined the effects of diallyl disulfide (DADS), one of the major components of oil-soluble garlic extracts (GE) and of raw water GE on SH-SY5Y and NSC34 neuronal cell lines. Both treatments with DADS and GE were able to induce growth arrest and apoptosis, and we observed an increased flux of reactive oxygen and nitrogen species as early signs of cytotoxicity. We demonstrated that the content of neuronal nitric oxide synthase (nNOS) increased as early as 1 h of treatment demonstrating to be a very early sensor of DADS and GE cytotoxicity. Treatments with L-nitropropyl-arginine, an inhibitor of nNOS, increased the rate of apoptosis whereas the overexpression of nNOS significantly reduced cell death by inhibiting DNA damage, protein oxidation, and the activation of the JNK/c-Jun apoptotic signaling cascade. Overall these results demonstrate that garlic derivatives may modulate nNOS and suggest an important contribution of nitric oxide in counteracting their reactive oxygen species-mediated cytotoxicity.