Many freshwater and marine animals, both vertebrate and invertebrate, are particularly sensitive to certain xenobiotics. Such organisms are used as models to identify those physiological processes which are most sensitive to environmental pollutants. Particular emphasis is placed on membrane functions which underly these processes. One such model system utilizes the blue crab, which is extremely sensitive to DDT and other organochlorine pesticides, particularly at low environmental salinity where its survival depends on the crab's ability to osmoregulate. Parameters examined include the sensitivity of 1) the Na ion, K ion ATPase to organochlorine, 2) the ion fluxes dependent upon this enzyme, and 3) the ability of the intact animal to maintain hemolymph osmolarity and ion levels.