The reactions of Rhus vernicifera (monophenol,dihydroxyphenylalanine: oxygen oxidoreductase, EC 1.14.18.1) with the reducing substrates hydroquinone and ascorbic acid have been investigated with the stopped-flow technique. Rhus laccase appears to be present in two molecular forms with a pH-sensitive equilibrium constant regulating the relative concentrations of each species. A model for the reaction of Rhus laccase with reducing substrates has been formulated. The model is similar to one formulated earlier for the anaerobic reduction of laccase from Polyporus versicolor (Andréasson, L.-E., Malström, B.G., Strömberg, C. and Vänngård, T. (1973) Eur. J. Biochem. 34, 434-439) and accounts for the reduction also of this enzyme. The essentials of the model are as follows: Electrons are taken up from reductants one at a time. The type 1 Cu2+ has a central role in mediating the transfer of at least one of the electrons needed for the reduction of the co-operative two-electron acceptor. Intramolecular reactions determine the concentrations of two molecular forms of the enzyme and influence the rate of reduction of the two-electron acceptor. The model, which has been used for successful simulations of the anaerobic reduction of Rhus laccase, is capable of explaining the reduction of laccases also in the presence of the inhibitor F-. In addition, the model gives an explanation of the behaviour of the laccases when reducing substrates and O2 are simultaneously present and is consistent with earlier observations of the post-steady-state reduction of the type 1 Cu2+ and the two-electron accetor (Holwerda, R.A. and Gray, H.B. (1974) J. Am. Chem. Soc. 96, 6008-6022).