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Freshwater fish (larval lampreys Petromyzon marinus) occupied plastic tubes and ventilated aqueous solutions (around 1.0 mg/L) of the organochlorine pesticide Kepone at 13 degrees C. The efficiency with which Kepone was extracted from solution during a single pass through the pharynx, and the effect of Kepone on fluxes of Na+ and Cl- across the gills were measured. The efficiency with which lampreys extracted Kepone from local tap water averaged 55-66%, confirmed by measuring Kepone accumulation within the lampreys (as 14C-equivalents), using a mass-balance approach. The extraction efficiency matches predictions based on studies using teleosts. Extraction efficiencies declined slowly throughout the exposure period and related inversely to ventilatory rates. In ion-flux measurements, Kepone produced a 40% decline in the rate of influx of Cl- across the pharynx. Effluxes of Na+ and Cl- were not affected, nor was influx of Na+. These latter findings are difficult to reconciliate with accepted theories of Kepone's toxic action; they may relate to the fact that lampreys are unusually tolerant to Kepone poisoning.