Schisandra fruits (Schisandraceae) are often used in traditional medicine and can be taken concomitantly with conventional medicine. In this study, the effects of dibenzocyclooctadiene lignans from Schizandra chinensis on P-gp-mediated efflux were examined to investigate a possible interaction with P-gp substrates. The cellular accumulation of rhodamine-123 in Caco-2 cells was measured with 12 Schisandra lignans. Most compounds resulted in slight or moderate increases of rhodamin-123 cellular uptake, indicating their P-gp inhibitory activity. Among them, deoxyschizandrin exhibited the most potent effect on the accumulation of rhodamine-123. Subsequently, bidirectional transports of digoxin and rhodamine-123 in Caco-2 cells were determined with deoxyschizandrin, the most active compound for the rhodamine-123 assay. In the bidirectional transport study, apical-to-basal (A-to-B) transports of digoxin and rhodamine-123 were increased, whereas basal-to-apical (B-to-A) transports were decreased by deoxyschizandrin in concentration- and time-dependent manners. At 50 microM of deoxyschizandrin, the transport ratios (B-A/A-B) for digoxin and rhodamine-123 were 2.2 and 2.1 compared with the control ratios of 15.2 and 12.2, respectively. These results demonstrated that deoxyschizandrin effectively inhibited the P-gp-mediated efflux in Caco-2 cells, suggesting they could potentially increase the absorption of drugs that can act as a P-gp substrate.