A number of tetrazole analogs of carboxylic substrates and inhibitors have been tested. Lactic and pyruvic tetrazoles were found to be competitive inhibitors of rabbit muscle L-lactate dehydrogenase in both the pyruvate reduction and the lactate oxidation reactions (Ki's of 0.04 M and 0.08 M D,L-lactic tetrazole and 0.02 M and 0.035 M pyruvic tetrazole, respectively). Lactic tetrazole is a non-competitive inhibitor of yeast L-lactate dehydrogenase (Ki = 0.10 M D,L-lactic tetrazole) while pyruvic tetrazole is predominantly competitive (Ki = 0.15 M). Alanine tetrazole is a poorer substrate than alanine for D-amino acid oxidase. It also acts as weak inhibitor. Benzoic tetrazole is a substrate-competitive inhibitor of D-amino acid oxidase (Ki = 0.7 mM) and is also a stronger ethanol-competitive inhibitor than benzoic acid (Ki = 0.03 M) of liver alcohol dehydrogenase. In all the substrates and inhibitors tested, substitution of a tetrazole ring for a carboxylic group has resulted in decreased binding, presumably due to a dilution of the negative charge density and the larger size of the tetrazoyl anion.