AbstractSubstrate oxidation was measured in the gills of blue crabs, Callinectes sapidus, acclimated to different salinities. The oxidation rates for glucose and the four major amino acids of hemolymph were about twice as great in the gills from crabs acclimated to 17%0 seawater relative to the gills from crabs acclimated to full‐strength, 35%0 seawater. Alanine oxidation contributed about 36% to the total CO2 released from amino acid oxidation. Alanine was also deaminated and released into the medium as α‐keto acids. Both alanine oxidation and α‐keto acid formation were concentration dependent and did not appear to reach saturation within physiological concentrations. However, the rates of amino acid oxidation were not sufficient to explain the efflux of NH4+ in excised gills.