The effects of 2-iodosobenzoic acid, 4-chloromercuribenzoate, 5,5'-dithiobis-(2-nitrobenzoic acid) and tetraethylthioperoxydicarbonic diamide (disulphiram) on the NAD+-dependent activity of xanthine oxidoreductase from rat liver were investigated. Only disulphiram converted the NAD+-dependent activity into the O2-dependent activity quantitatively, without changing the xanthine hydroxylation rate. The modification process was a first-order reaction with respect to time (min) and disulphiram concentration (microM). The kinetic data showed that modification of single thiol group is sufficient for loss of the enzymic activity towards NAD+ as electron acceptor. The complete protection afforded by NAD+ against the action of disulphiram suggests that the essential thiol group may be involved in binding of NAD+ to the xanthine oxidoreductase molecule.