1.β-Aminolaevulic acid (ALA) dehydrase activity has been measured in the blood of man and rats and in the tissues of rats intoxicated with ethanol. 2. ALA dehydrase activity was significantly depressed in the blood of man. This depression follows closely on the elevation of blood ethanol and returns to normal pari passu with the ethanol level. 3. In rats intoxicated with ethanol there is a significant depression of ALA dehydrase activity in blood, liver and kidney but not in heart or spleen. 4. A scheme is proposed whereby an increase in the intracellular redox potential by ethanol increases sulphydryl cofactor concentrations leading to an inhibition of enzyme activity.