Administration of the organophosphate insecticide, chlorpyrifos to immature rats exerted a proconvulsant effect on seizures induced by kindling. Chlorpyrifos was administered to 16 or 17 day old rats in a dose range of 0.3 to 10 mg/kg, subcutaneously. Amygdala kindling was performed by stimulating the rats every 15 minutes to a total of 20 stimulations. Kindling occurred more rapidly in the chlorpyrifos treated rats than vehicle treated rats, the proconvulsant effect was dose-dependent. The proconvulsant effect of chlorpyrifos was more pronounced in the early stages of kindling, indicating a possible increase in local excitability of the amygdala in the presence of chlorpyrifos. Chlorpyrifos also reduced the after discharge threshold in the amygdala in a dose-dependent manner and increased the duration of after discharges elicited by electrical stimulus, indicating an increase in excitability of the amygdala. The effects of chlorpyrifos on kindling were additive with xylene: the proconvulsant effect in the early stages of kindling was greatly enhanced by xylene. Xylene, administered alone as a 0.2% solution, reduced the after discharge threshold of the amygdala, increased the after discharge duration and increased the rate of kindling. These experiments demonstrate a proconvulsant effect of chlorpyrifos in amygdala kindling and this proconvulsant action is additive with xylene.