Yeast alcohol dehydrogenase (EC 1.1.1.1) catalyzes the novel reduction of p-nitro-so-N,N-dimethylaniline with NADH as a cofactor. Apparent kinetic constants for this enzymatic reaction are: V-2=2.1 s(-1), K-Q=456 muM, K-iQ=119 muM, and K-P=1.47 mM, at pH 8.9, 25 degreesC. This reaction is especially useful for the quantitative determination of NAD(+) and NADH by enzymatic cycling.