The effects on behavior and cholinesterase (ChE) of an OP pesticide, dimethoate, were examined in wood mice under laboratory conditions. Mice were administered 0, 5, 15, or 50 mg/kg intraperitoneal dimethoate and their behavior was recorded in an open field for one hour. In a second experiment, using only the 0 and the 50 mg/kg dose, mice were subjected to 10-min open field tests repeated at various time intervals during a 24-h period. Shortly after administration of dimethoate, there was a general, dose-dependent, behavioral depression that was characterized by increased inactivity and decreased grooming, rearing, and sniffing. The introduction of a novel object in the open field failed to elicit any reaction in mice treated with the two highest doses of dimethoate. The behavioral impairment completely disappeared 6 h after treatment. A stereotyped compulsive grooming was also observed in the first 30 min after administration of the two highest doses. Exposure to dimethoate caused a dose-dependent decrease in ChE activity in the brain and in serum. Behavioral impairment was associated with maximum levels of ChE inhibition of 65-75% (brain) and 75-85% (serum). Recovery of ChE activity lagged behind that of behavioral impairment and started 3-6 h after dimethoate administration. The possible implications for free-living wood mice which inhabit cereal fields and may be exposed to OPs are discussed.