The tests described were designed to test the hypothesis that (1) lapeses of attention-whether due to 'blanking out,' 'inappropriate sense of well-being,' or other causes-are relatively common in persons with extensive exposure to organic phosphorus insecticides, but (2) these lapses come to attention only when they are sufficiently prolonged and occur during critical situations, such as the operation of aircraft. The alertness tests used in this study were selected primarily on the basis of their capability to measure lapses of attention. Also, they give some measure of other altered mental functions, such as decreased judgment, although their sensitivity for this purpose is less than for determining lapses of attention.