The inactivation of low-KM rat liver mitochondrial aldehyde dehydrogenase (ALDH) by the alcohol-sensitizing agent cyanamide (H2NCN) has been studied in vitro. The effect of the concentrations of NAD+ at different concentrations of catalase on the inactivation of ALDH by cyanamide (20 and 200 microM) in vitro point to an ALDH-NAD(+)-catalase complex prior to the binding to cyanamide to form the holoenzyme-inhibitor complex. Cyanamide itself could be responsible for the inactivation of ALDH. The possibility that both irreversibly inactivated ALDH and cyanamide remain free at the end of the inactivation process is discussed. The effects of pH and ionic strength on the inactivation process are also described. The pseudo-first order rate constants for inactivation of low-KM ALDH depends on both effects, suggesting that electrostatic forces are involved in the process and that a group with pK approximately 6.8, presumably a histidine residue, at the active site of ALDH could be involved. A representative equation for the inactivation process of low-KM ALDH by cyanamide in vitro has been fitted to experimental kinetic data, involving both catalase and inhibitor concentrations.