Depression and other mood disorders, as well as cognitive and motor dysfunction have been linked with changes in monoamine levels in the brain. Environmental acetylcholinesterase (AChE) inhibitors, such as organophosphate insecticides (OPs), have also been shown to induce these problems. This study investigated whether insecticide-induced AChE inhibition, induced by chlorpyrifos (CPS), may contribute to the types of forebrain monoaminergic alterations associated with psychiatric, cognitive and motor dysfunction. Increased synaptic ACh, resulting from CPS-induced AChE inhibition, may alter the synthesis or release of monoamines through prolonged action of ACh on monoaminergic neurons that contain ACh receptors. Adult, male Sprague-Dawley rats were subjected to a single subcutaneous dose of CPS or corn oil vehicle. Brains were rapidly removed and the frontal cortex, hippocampus and striatum were bilaterally dissected on ice. These three regions from one side were assayed for AChE activity, while those from the opposite side were processed for high performance liquid chromatography with electrochemical detection (HPLC-ED) analysis of monoamine neurotransmitters and their metabolites. In the initial, exploratory experiment, inhibition of AChE activity was 66.8% in the frontal cortex, 43.8% in the hippocampus and 46.9% in the striatum, 7 days after a 60mg/kg dose of CPS. No significant differences in concentration of monoamine neurochemicals were observed between vehicle control and CPS-treated groups in either the hippocampus or striatum. However, in the frontal cortex of the CPS-treated rats there was a significant increase in median dihydroxyphenylacetic acid (DOPAC) concentration (P=0.019) and a very strong statistical trend toward increased dopamine (DA) concentration (P=0.0506). The second experiment examined the time course of AChE inhibition produced by a higher dose (200mg/kg) of CPS and how monoamine levels changed in conjunction with this pattern of AChE inhibition. Percent inhibition of AChE activity in CPS-treated animals, at 4, 14 and 21 days post-exposure was 77.0%, 86.6% and 81.9% in the frontal cortex, 86.1%, 85.9% and 83.2% in the hippocampus and 90.1%, 89.8% and 85.5% in the striatum. No significant differences in monoamine neurochemicals were observed between vehicle control and CPS-treated groups in either the hippocampus or striatum. A statistical trend toward a decrease in serotonin (5-HT) was seen in the frontal cortex at 14 days (P=0.0753) following CPS exposure. A very consistent, yet non-significant pattern of an increase in monoamines at 4 days post-CPS was observed in all instances, except for 5-hydroxyindoleacetic acid (5-HIAA) in the striatum. Therefore, the final experiment employed a more powerful design to focus on monoamine levels during, or shortly after, the change in AChE activity that rapidly follows exposure to 200mg/kg CPS. This experiment also employed a behavioral analysis on the day of sacrifice to assess the presence or absence of clinical signs of toxicity associated with this dose. Of the 30 CPS-treated rats, only 1 animal displayed a single behavioral sign of cholinergic poisoning. Percent inhibition of AChE activity at 2 and 4 days after treatment was 81.4% and 79.4% in the frontal cortex, 53.4% and 83.5% in the hippocampus, and 80.5% and 87.8% in the striatum. No significant changes in monoamine neurochemicals were observed between vehicle control and CPS-treated groups in either the frontal cortex or hippocampus. However, a significant increase in DOPAC (P=0.0285) in the striatum, 2 days after CPS treatment, was observed. In addition, a strong statistical trend toward decreased striatal 5-HT (P=0.0645) was reported 4 days after CPS treatment. The only significant correlation between AChE activity and monoamine concentration was observed for 5-HIAA in the striatum of CPS-treated, 2 day survivors (P=0.0445). However, it was of low magnitude (r=0.525, r2=0.276). CPS has a limited capacity to produce changes in monoamine neurotransmitters and/or their metabolites in the frontal cortex and striatum of the mammalian brain. These changes are primarily seen in the dopaminergic system. Alterations of monoamines do not appear to be strongly associated with incident levels of AChE inhibition. The biological implication of the limited OP induced changes in central monoamines remains significant, as changes in monoamines in the CNS nervous system have been linked to psychiatric, cognitive and motor dysfunction.