Serum paraoxonase-1 (PON1) is an esterase associated with high-density lipoproteins in plasma and is involved in the detoxification of organophosphates (OP). We have previously reported a significant decrease in serum PON1 activity following Nippostrongylus brasiliensis infection in Wistar rats. In the present study we investigated the effects of decreased serum PON1 activity due to N. brasiliensis infection on acute toxicity induced by chlorpyrifos oxon (CPO) and paraoxon (PO) in rats. CPO and PO were dermally applied at doses of 8 mg/kg and 0.2 mg/kg body weight, respectively, to infected (on day 7 post-infection) and uninfected rats, after which acetylcholinesterase (AChE) activity was measured within the brain, diaphragm, plasma, and red blood cells, 4h after administration as a measure of toxicity. In addition, serum PON1 activity was measured immediately prior to administration of CPO and PO. N. brasiliensis infection significantly increased the degree of inhibition of AChE in the brain and diaphragm after treatment with CPO and PO in association with a significant reduction in PON1 activity. Likewise, similar findings were observed in the blood (plasma and RBCs) ChE activity after treatment with PO, but not CPO. These results indicate that N. brasiliensis infection makes rats more susceptible to CPO and PO toxicity, suggesting that gastrointestinal nematode infection might be a potential factor affecting OP toxicity.