Arsenic is a heavy metal that is ubiquitous in the environment. The toxicity of arsenic depends upon its chemical form; the organic forms being usually less harmful than inorganic ones. The primary source of human exposure is through drinking water and food. Arsenic acts on cells through a variety of mechanisms, influencing numerous signal transduction pathways and resulting in a vast range of cellular effects that include apoptosis induction, growth inhibition, promotion or inhibition of differentiation, and angiogenesis inhibition. The primary objective of this research is to evaluate the effects of arsenic trioxide on DNA synthesis and to determine whether arsenic induces apoptosis via caspase activation and the activation the mitogen -activated protein kinase (MAPK) in lung carcinoma cells. To achieve this goal, the lung cancer (A549) cells were cultured following standard protocols, and exposed to various doses (0, 2, 4, 6, 8, and 10 mug/ml) of arsenic trioxide for 48 h with LC(50) being 7.8mug/ml. The proliferative response (DNA synthesis) to arsenic trioxide was determined by [(3)H] thymidine incorporation assay. Arsenic trioxide-induced apoptosis was determined by DNA laddering. Caspase -3 activation was assessed by the caspase-3 fluorescein isothiocyanate (FITC) assay. p38 MAP kinase activity was examined by immunoblot analysis using phospho p38 MAPK mab primary antibody in the presence of ATP and transcription factor (ATF-2) as a substrate. [(3)H] thymidine incorporation assay revealed biphasic reaction; showing cell proliferation at a lower level of exposure, and a dose-related cytotoxic response at higher levels of exposure in A549 cell line. Findings from the DNA laddering assay indicated that arsenic trioxide induced apoptosis in the lung carcinoma cells. Our findings revealed that arsenic trioxide modulated caspase 3 activity and induced p38 map kinase activation in lung carcinoma (A549) cells.