To provide data for the establishment of criteria for evaluating a safe re-entry period for farm workers after the application of organophosphorus pesticides such as guthion, parathion, methyl-parathion as well as other organophosphates which are widely used in California for pest control. To account for the fate of organophosphorus pesticides applied to selected crops, in terms of pesticide's disappearance from plant surfaces under typical field conditions. Initially we will take crops on which large quantities of organophosphorus pesticides are presently being used in California. During the first year we will study peaches and in subsequent growing seasons, we intend to study other important vegetable crops and grapes when feasible. These crops usually require substantial field-worker exposure during thinning and harvesting operations. To measure physiological effects on workers during their normal work activities in fields that have been treated with organophosphorus pesticides. Although blood cholinesterase depression is a general index for organophosphate and carbamate poisoning, poisoning can also be measured by the metabolite in the urine. For example, in the case of parathion, by measuring p-nitrophenol one would avoid interference from most other cholinesterase inhibitors.