For certain CYP3A4 substrates intestinal first-pass metabolism makes a substantial contribution to low oral bioavailability and extent of drug-drug interactions (DDI). In order to include the contribution of enzyme inhibition in the gut wall in the assessment of DDI potential, the ratio of the intestinal wall availability in the presence and absence of an inhibitor (F(G)(') and F(G), respectively) has been incorporated into a prediction equation based on hepatic enzyme interactions. This approach has been applied for both reversible and irreversible DDIs, involving 36 different inhibitors and 11 CYP3A4 substrates. The aim was to investigate the use of maximal (complete) inhibition of intestinal CYP3A4 (F(G)(')=1) as a pragmatic measure of the intestinal enzyme interaction and to compare this approach with observed in vivo values (where available) and predicted F(G) ratios from an intestinal model. The latter was obtained from the decrease in the intestinal intrinsic clearance in the presence of an inhibitor, using an estimated inhibitor concentration in the intestinal wall during absorption phase (I(G)) and an in vitro obtained K(i). In addition, the impact of variability in the enterocytic blood flow on the estimated I(G) and subsequently the model predicted F(G) ratio was investigated. The maximal F(G) ratios for the 11 CYP3A4 substrates investigated ranged from 1.06-7.14 for alprazolam and tacrolimus, respectively. In 91% of the studies investigated the model predicted F(G) ratio was within 40% of the maximal value. Maximal F(G) ratio is proposed as an initial indicator of the magnitude of intestinal enzyme interaction; the implications for drug elimination involving substrates cleared either by metabolism or by a combination of metabolism and efflux transporters are discussed.