Abstract 1. 1. Lactate dehydrogenase (LDH) affinity for pyruvate is twofold to threefold higher at pH 6·0 than at pH7·9 in the case of swimbladder and lung LDHs. 2. 2. For the swimbladder LDH, but not the lung LDH, optimal velocity and substrate inhibition are higher at pH 6·0 than at pH 7·9. 3. 3. These effects of pH appear to be adaptive.