Involuntary exposure to environmental tobacco smoke (ETS) in public or in working places is considered to be a serious risk to human health. This symposium addressed several issues of toxicological interest that are associated with exposure to ETS. Epidemiologic evidence obtained in human studies suggests that "passive smoking" increases the risk of developing lung cancer in nonsmokers and favors the development of respiratory tract infections in children. Comparatively few data are available from animal studies that provide experimental support of the observations. Exposure of pregnant or neonate rats to cigarette sidestream smoke (SS) affects developmental patterns of drug metabolizing enzymes that may persist up to 90 days. In young roosters, SS accelerates the development of arteriosclerotic plaques. On the other hand, exposure of adult rats for up to 90 days induces only transient signs of damage in the nasal passages, but not in the deep lung, and this only at extremely high concentrations of ETS. So far, experimental toxicology has provided comparatively few data on the correlation between exposure to ETS and adverse health effects. yet, such data are needed, particularly since many conclusions drawn from the epidemiological studies remain open to criticism and questions.