Extensive cotton producing areas of California suffer from air pollution. There is reason to believe that this problem will increase in severity and extent with increases in industrial and urban development. Premature aging and dropping of leaves of susceptible plants are known to be caused by natural and experimental, photochemically produced, compounds in smog. Ozone and peroxy-acetyl nitrate (PAN), two known phyto-toxicants in smog, have been demonstrated to produce such leaf separation under laboratory conditions.