The paper reviews some of the studies that have been done, to evaluate the potential hazard to human health from occupational and environmental exposure to pesticides. A number of studies of the oral toxicity of various organic phosphorus insecticides have been made using human volunteers. These tests have generally been of relatively short duration. Inhibition of flood cholinesterase activity serves as a sensitive measure of incipient toxic effect in tests of this sort. Other volunteer studies have been primarily oriented toward measurement of effect of insecticides on some specific physiological function, such as vision or behavior. A considerable amount of data is available on the magnitude of exposure of workers and residents to pesticides during application for agricultural or public health purposes. A smaller amount of information is available on the relationship of this measured occupational or environmental exposure to the development of clinical signs or symptoms of poisoning or of demonstrable abnormalities of physiological function. There is a need for extending application of the presently available techniques for measurement of exposure to other compounds and to other occupational and environmental situations.