Human pentavalent arsenic metabolic intermediate, monomethylarsonic acid (MMAs V), is a major arsenic type found in the blood in chronic arsenic poisoning patients, but little information is available on its toxicity potential or mechanisms of action. In this study, we investigated the molecular mechanisms of in vitro cytolethality of MMAs V using rat liver TRL 1215 cells. Cellular arsenic concentrations reached the nanomolar range in TRL 1215 cells when cells were exposed to millimolar levels of MMAs V, and most of the MMAs V was not metabolized during the 48-h incubation. Under these conditions, MMAs V showed significant cytolethality when cellular reserves of reduced glutathione (GSH) were depleted. Morphological and biochemical evidence confirmed that MMAs V induced both necrosis and apoptosis in the cellular GSH-depleted cells. MMAs V significantly enhanced cellular caspase 3 activity in the cellular GSH-depleted cells, and a caspase 3 inhibitor blocked MMAs V-induced apoptosis. MMAs V also enhanced the production of cellular reactive oxygen species (ROS) in the cellular GSH-depleted cells, and addition of a membrane-permeable radical trapping reagent completely prevented both MMAs V-induced cellular caspase 3 activation and cytolethality in these cells. These observations suggest that MMAs V typically generates harmful ROS in cells, and cellular GSH prevents cytolethality by scavenging these toxic ROS. However, when cellular GSH levels are decreased, MMAs V induces oxidative stress in the cells, and this leads to apoptosis and/or necrosis depending on the cellular ROS/GSH ratio.