Despite the fact that it is an aldehyde, glyoxylic acid is not a substrate for sheep liver cytoplasmic aldehyde dehydrogenase; instead it functions as an inhibitor of both the esterase and dehydrogenase activities. From a consideration of the inhibition patterns it is concluded that glyoxylic acid does not bind in the catalytic propionaldehyde-binding domain, thus confirming the two-site model as proposed previously. Since the corresponding neutral methyl ester is a substrate it is suggested that the catalytic binding domain must contain a negatively charged group which prevents the binding of glyoxylic acid. Steady-state and pre-steady-state kinetic studies indicate that glyoxylic acid inhibits the dehydrogenase activity by converting the enzyme into a dead-end form which cannot undergo the catalytically essential conformational change. Incubation of the enzyme with NAD+ and glyoxylic acid for 10 min before the addition of propionaldehyde gave rise to hysteresis effects which can be explained on the basis of a slow isomerization of the enzyme X NAD+ X glyoxylic acid complex.