Background: Cigarette butts are the most common form of litter in the world;rnapproximately 5.6 trillion cigarettes are smoked yearly worldwide. Over 7000 chemicalsrnmay be introduced to the environment in cigarette butts and smoke constituents. Theserninclude chemicals such as nicotine, carbon monoxide, hydrogen cyanide, polycyclicrnaromatic hydrocarbons, heavy metal ions, nitrosamines, ammonia, acetaldehyde,rnformaldehyde, benzene, phenol and acetone, over 70 of which are carcinogenic tornhumans. Environmental cigarette butts are toxic to microorganisms, fish, other marinernorganisms and birds, but no studies have been done on their toxicity to mammals.rnMethods: In this study the toxicity of cigarette butt leachates to Swiss albino mice wasrninvestigated, and the effects of cigarette butts with and without associated tobacco wererncompared. Cigarette butt leachates were made by soaking different numbers of cigaretternbutts in 250 mL of tap water overnight, and leachates were given to mice to drink. Thernweights of mice as well as amounts of food and fluids consumed were followed overrntime. Necropsies were performed and tissues were weighed and evaluated by histologicalrnstaining with eosin and hematoxylin. Blood glucose and liver function tests (ALT, ASTrnand ALP) were also measured.rnResults: Mice given high-concentration cigarette butt leachates during their growth fromrn6 weeks to 16 weeks preferred water over any of the leachates, whereas mice given lowerrnbutt concentrations of leachates representative of the relative frequencies of butt types inrnthe environment drank butt leachates as well as water. Food intake was not affected byrncigarette butt leachates. Mean fasting blood glucose and activities of serum liver enzymesrnwere not different between mice given leachates or water to drink. The weights of thernmice, as well as tissues (liver, lung, spleen, heart, kidney, epididymal fat pads) werernsignificantly lower in mice that drank leachates with associated tobacco than in mice thatrndrank water alone or leachates made from butts without associated tobacco. This suggestsrnthat a component(s) of tobacco is/ are responsible for reducing weight gain during mouserngrowth. Cigarette butt leachates had no effect on histopathology of liver, heart andrnkidney, whereas lung tissue of the mice that drank leachates made with tobacco-rnassociated butts showed increased air space volumes and alveolar fibrosis, compatiblernwith pulmonary emphysema.rnConclusions: Cigarette butts are toxic to laboratory mice, causing reduced weight gainrnduring growth, reduced mass and size of tissues and organs, and pulmonaryrnemphysematous changes. An average cigarette butt thrown into the environment containsrn1.5 mg of nicotine, enough to potentially kill 15 mice. The implications of these studiesrnto environmental animals in Ethiopia and elsewhere are discussed