Rats were administered the organophosphorus insecticide acephate at 1.0 or 10.0 mg/kg · day for 15 weeks. Blood and brain samples were collected at the end of the treatment and analyzed for cholinesterase, acetylcholinesterase, and glutamic acid decarboxylase activities and catecholamine and amino acid levels. No significant inhibition in the activity of brain AChE was noted at doses of 1.0 or 10.0 mg/kg · day. Low levels of acephate exposure (1.0 mg/kg · day), which did not alter plasma cholinesterase or RBC acetylcholinesterase activity levels, resulted in a significant elevation of plasma epinephrine and norepinephrine levels. Decreased GABA, dopamine, and tyrosine levels and glutamic acid decarboxylase activity were observed in brains of these rats. Similar changes occurred in rats exposed to 10 mg of acephate/kg · day; however, plasma cholinesterase and RBC acetylcholinesterase activities were inhibited. These observations suggest that chronic exposure to acephate altered the activity of the noncholinergic system without altering the cholinergic activity, and that low-level chronic exposure to organophosphorous compounds cannot be predicted by measuring the ChE or AChE enzyme activities.