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1. 1. Na + ,K + -ATPase, which mediates the active transport of Na + and K + across the plasma membrane, is found in equivalent amounts in both plasma membranes of the electrocyte, the anterior, non-innervated (fraction P 2 ) and the posterior, innervated (fraction P 3 ) obtained by differential centrifu gation of Electrophorus electricus (L.) electric organ. 2. 2. The kinetic effects of Hg 2+ and A1 3+ , described as neurotoxic metals, on the Na + ,K + -ATPase activity of the two membrane fractions (P 2 and P 3 ) were analysed with respect to Na + and K + ions, after the I 50 estimation of each metal. 3. 3. Mercury is a potent Na + ,K + -ATPase inhibitor in the nanomolar range. In all cases, it behaved as a mixed partial hyperbolic inhibitor. 4. 4. Aluminum was shown to be a poor enzyme inhibitor. Changing the K + concentration, it behaved as a mixed linear inhibitor (P 2 fraction) and as a non-essential mixed activator (fraction P 3 ). Aluminum behaved as a partial hyperbolic inhibitor for both P 2 and P 3 fractions with respect to Na + concentration. 5. 5. The observation of the variable kinetic behaviour of P 2 and P 3 led us to attribute these differences to the Na + ,K + -ATPase electrocyte isoenzymes which occur in different proportions in these fractions (Gomes-Quintana et al. , 1992 Comp. biochem. Physiol. 103B/3 623–628).