In Arunachal Pradesh and other sub-Himalayan areas of India, accidental consumption of Senecio plants by yaks is often fatal as the plant contains toxic alkaloids like Seneciophylline. The present investigation was undertaken to demonstrate the pro-oxidant effects of an ethanolic extract of Senecio chrysanthemoides (S-EtOH). S-EtOH impaired viability in macrophages, the IC(50) being 13.8+/-1.11 microg/mL. The effect of S-EtOH (1 microg/mL) on generation of reactive oxygen species (ROS) in macrophages was measured by flow cytometry using 2',7'-dichlorofluorescein diacetate (H(2)DCFDA) where it caused a significant increase in the mean fluorescence channel (MFC) from 8.55+/-0.03 to 47.32+/-2.25 (p<0.001). S-EtOH also effected a 3.8-fold increase in extracellular nitric oxide (NO) generation from 4.90+/-0.72 microM to 18.79+/-0.32 microM (p<0.001), a 2.2-fold increase in intracellular NO production, the MFC increasing from 14.95+/-0.48 to 33.34+/-1.66 (p<0.001), and concomitantly depleted non protein thiols as analyzed by flow cytometry using mercury orange, with a reduction in MFC from 632.5+/-49.44 to 407.4+/-12.61 (p<0.01). Additionally, S-EtOH (14 microg/mL, 24h) caused apoptosis as evident by increased Annexin V binding and terminal deoxynucleotidyl transferase mediated dUTP DNA nick end labeling. Taken together, the cytotoxicity of S-EtOH can be partly attributed to its capacity to inflict oxidative damage via generation of both reactive oxygen and nitrogen species culminating in apoptosis.