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Reduction in cholinesterase activity levels of various tissues (blood, brain) is one of the earliest signs of chlorpyrifos intoxication. Cholinesterase reductions have been demonstrated in turkeys fed diets containing 50 mg chlorpyrifos/kg (estimated daily dos,e of 0.7 mg/kg BW) for 20 days (Schlinke et al. 1969),. in chickens fed diets of 25 mg/kg (estimated daily dose of 0.94 mg/kg BW) for 20 days (Schlinke 1970), and in mallard (Anas platyrhynchos) ducklings fed 75 mg chlorpyrifss/kg diet for 14 days (Herin et al. 1978). Low temperatures (27.5 Oc vs. 35 C) potentiated dose-related ChE depression in juvenile nothern bobwhite (Colinus virginianus), suggesting a need for more research on cold stress interactions between acute oral chlorpyrifos exposure (Maguire and Williams 1987). Dietary concentrations of 30 to 100 mg chlorpyrifos/kg feed produce some deaths in birds, and 136 to about 500 mg/kg feed usually kills at least 50% (Table 6). In chickens fed diets of 100 mg chlorpyrifos/kg--equivalent to an estimated daily dose of 6.8 mg/kg 8W--egg fertility was reduced by 15% and hatchability by 17% (Schorn et al. 1973). Dietary levels lethal to mallard ducklings were 136 to 180 mg/kg feed, equivalent to 10 mg/kg BW fed daily for 5 days (Kenaga 1974). In adult mallards given diets containing 80 mg chlorpyrifos/kg for 60 to 84 days, body weight, food consumption, brain cholinesterase actvity levels, and egg production were all reduced; moreover, egg weight and eggshell thickness were reduced, the resultant ducklings weighed less than controls, and survival was comparatively poor at age 7 days. No effect on any variable was observed at diets of 8 mg/kg (Gile and Meyers 1986; Meyers and Gile 1986). Dermal application routes are also toxic. Some deaths were recorded in turkeys from dermal treatments of 15 to 20 mg chlorpyrifos/kg BW (Schlinke et al. 1969). Higher levels applied to feathers killed turkeys within 8 hours (Marshall and Roberts 1978). Newborn piglets (Sus spp.) were especially more sensitive than those 30-36 hours old to cutaneous applications of chlorpyrifos; newborns showed clinical signs consistent with organophosphorus toxicosis after a 2.5% aerosol preparation (dosage unknown) was applied to the tail and umbilicus (Long et al. 1986). Accidental poisoning of cattle (80S spp.) by chlorpyrifos through dermal application to control ticks resulted in some deaths; among bulls that survived, sperm production was reduced 43% in . seriously affected animals and 12% in those with no outward signs of poisoning (Everett 1982). Chlorpyrifos is not mutagenic, as judged by mitotic recombination assays (Poole et al. 1976), and did not increase sister chromatid exchange above background in tests with chick (Gallus spp.) embryos and Chinese hamster (Cricetus spp.) ovary cells (Muscarella et al. 1984). 12