Channel catfish, Ictalurus punctatus (Rafinesque), were exposed to continuously renewed solutions of endrin in water. Analyses of the fish blood by gas chromatography revealed a well-defined threshold concentration of endrin in the blood, approximately 0.30 microgram per gram, that, if exceeded, results in death. Fish exposed to lethal concentrations of endrin in water for periods of time insufficient to cause death had blood-endrin concentrations markedly lower than those that died from exposture to the same water. There was little overlap in range of endrin concentration in blood between dead and living exposed fish.