Each year for nearly 20 years, thousands of pounds of persistent organochlorine pesticides have been applied to outdoor areas in many countries. These compounds may last for a very long time in the environment, and be carried by wind, water, and animals to places far distant from where they are used. As a result, most living organisms now contain organochlorine residues. This paper constitutes a selective review of the literature concerning the occurrence, distribution, and effects of organochlorines in the environment. Highest concentrations generally occur in carnivorous species. Thus predatory and fish-eating birds ordinarily have higher residues than do herbivores; quantities are similar in birds of similar habits in different countries. Any segment of the ecosystem - marshland, pond, forest, or field - receives various amounts and kinds of pesticides at irregular intervals. The different animals absorb, detoxify, store, and excrete pesticides at different rates. Different degrees of magnification of pesticide residues by living organisms in an environment are the practical result of many interactions that are far more complex than implied by the statement of magnification up the food chain. These magnifications may be millions of times from water to mud or only a few times from food to first consumer. Direct mortality of wild animals as an aftermath of recommended pesticide treatments has been recorded in the literature of numerous countries. However, accidents and carelessness also accompany pesticide use on a percentage basis and are a part of the problem. More subtle effects on the size and species composition of populations are more difficult to perceive in time to effect remedies. The possibility of ecological effects being mediated through changes in physiology and behavior has received some attention and has resulted in some disquieting findings. These include discovery of the activity of organochlorines in stimulating the breakdown of hormones or in acting directly as estrogens, their involvement in embryonic and early post-embryonic toxicity, interferences with antibody formation, effects on behavior, and interactions with stress such as nutritional deficiencies or food deprivation. Delayed mortality long after dosage ceased has shown the serious effects of storage of organochlorines in fat. DDT has been suggested as the indirect cause of a reduction of egg-shell thickness that occurred in the midforties in association with failing reproduction and population decline of certain predatory birds. The impact of these new components of the environment has appeared as death, reproductive impairment, disruption of species balance, and behavioral alteration, but the overall effects on the environment have not been determined. Research should be aimed at interpretation of the significance of pesticide residues to survival and reproduction, to assessment of levels of pesticides in critical environments, and to the kinetics of pesticides in individuals and the ecosystem.