The biosynthesis of lactate from pyruvate has been used as a model system to demonstrate the applicability of electroenzymatic membranes as electron transfer chain biomimetics. A key feature is the in situ regeneration of the coenzyme NAD(H). This electro-enzymatic reaction utilizes an immobilized enzyme system (lipoamide dehydrogenase (LipDH) and methyl viologen as a mediator) within porous graphite cathodes, encapsulated by a cation exchange membrane (Nafion(R) 124, Dupont). The cathodic electrolyte contains the reaction mixture (pyruvate/lactate and co-products), the enzyme lactate dehydrogenase (LDH) and the co-enzyme NADH/NAD(+) system. Lactate yields up to 70% have been obtained when the reactor system was operated in a semi-batch (i.e. recirculation) mode for 24 h, as compared to only 50% when operated for 200 h in simple batch mode. The multipass, dynamic input operating scheme permitted optimization studies to be conducted on system parameters. These include concentrations of all components in the free solution, flow rates, and electrode configuration, i.e. its composition and transport characteristics. By varying system hydrodynamics in these membrane reactor systems, operating regimes that determine the controlling mechanism for process synthesis, i.e. transport versus reaction limitations, are readily identified. Procedures for developing operational maps are thus established.