This, according to theory based on the tubes-in-series model, was consistent with perivenous enrichment of glycination activity when transport of drug was even and when the ratio of drug influx/efflux coefficient exceeded that for metabolite. Similar benzoate transport in periportal, homogeneous and perivenous isolated rat hepatocytes existed, and the influx/ efflux coefficients (partition ratio) of benzoate from MID indeed exceeded that of hippurate. However, metabolism by zonal hepatocytes failed to reveal the anticipated metabolic zonation, and this is likely due to the shallow gradient of metabolic activity. The study demonstrates that moment theory is useful in delineating the perivenous enrichment of glycine conjugation activity.Drug removal by the liver involves the microcirculation, binding-debinding, transport, metabolism, and excretion, and is a distributed-in-space phenomenon that is modulated by zonation. A common method for probing enzyme zonation in the absence of a transport barrier is single-pass prograde/ retrograde perfusion of the rat liver preparation (Pang and Terrell, 1981;St-Pierre et al., 1989). With prograde perfusion, perfusate enters the portal vein and exits at the hepatic vein, first recruiting upstream and then downstream enzymes. With retrograde perfusion, flow enters the hepatic vein and exits the portal vein such that zonal distributions of enzymes appear reversed (Pang and Terrell, 1981;Pang et al., 1983). A more elaborate perfusion protocol is the dual perfusion of the hepatic artery (HA) and portal vein, or the hepatic vein (HV). If the substrate is delivered via HA singlepass to the liver with blank perfusate flowing retrogradely (HAHV), substrate will be confined to the periportal (PP) space. Conversely, with prograde flow of blank perfusate entering the portal vein (HAPV), substrate delivered from HA is swept across the entire liver (Pang et al., 1988). The removal rate constants for the respective regions may be obtained upon normalization of the metabolic activity to the cellular water spaces accessed (Chiba et al., 1994).By contrast, assessment of metabolic heterogeneity behind a transport barrier in the intact liver is more complex. The metabolic rates will depend on the transport of the drug and metabolite and whether the enzyme distribution is periportal (PP) or perivenous (PV). It is known that if transport across the hepatocyte membrane is poor, the fate of formed and preformed metabolites may differ (Sato et al., 1986;deLannoy and Pang, 1987;Schwab and Pang, 1999). Although there exist a few reports on the use of metabolite data to assess enzyme zonation after pulse injection of drug to the