Three organophosphate compounds, dichlorvos, parathion and diisopropylfluorophosphate were tested as an unconditioned stimulus in the conditioned taste aversion (CTA) test. All organophosphates caused a dose-dependent CTA in rats at doses which did not induce any other signs of toxicity. Experiments with dichlorvos showed that the minimum dose which caused CTA did not alter the rats' sensitivity to pain or their behavior in either an open field or an inclined plane. Cholinesterase activity was inhibited in a dose-dependent manner in brain and plasma after administration of the organophosphates and CTA was correlated with the degree of plasma cholinesterase inhibition. CTA appears to be a sensitive indicator of neurobehavioral effects of mild exposure to organophosphates which causes only 30-40% inhibition of plasma cholinesterase.