A review. The biochem. modification of the metals and metalloids mercury, tin, arsenic, antimony, bismuth, selenium, and tellurium via formation of volatile metal hydrides and alkylated species (volatile and involatile) performs a fundamental role in detg. the environmental processing of these elements. In most instances, the formation of such species increases the environmental mobility of the element, and can result in bioaccumulation in lipophilic environments. While inorg. forms of most of these compds. are well characterized (e.g., arsenic, mercury) and some of them exhibit low toxicity (e.g., tin, bismuth), the more lipid-sol. organometals can be highly toxic. Methylmercury poisoning (e.g., Minamata disease) and tumor development in rats after exposure to dimethylarsinic acid or tributyltin oxide are just some examples. Data on the genotoxicity (and the neurotoxicity) as well as the mechanisms of cellular action of organometal(loid) compds. are, however, scarce. Many studies have shown that the prodn. of such organometal(loid) species is possible and likely whenever anaerobic conditions (at least on a microscale) are combined with available metal(loid)s and Me donors in the presence of suitable organisms. Such anaerobic conditions can exist within natural environments (e.g., wetlands, pond sediments) as well as within anthropogenic environmental systems (e.g., waste disposal sites and sewage treatments plants). Some methylation can also take place under aerobic conditions. This article gives an overview about the environmental distribution of organometal(loid) compds. and the potential hazardous effects on animal and human health. Genotoxic effects in vivo and in vitro in particular are discussed.