This study investigates P-gp activity in placental villous fragments and the possibility of upregulating its expression and function by retroviral transduction. In fresh fragments, cyclosporin A caused a significant increase in 3H-vinblastine accumulation (187+/-48% at 180 min n=4), consistent with multi-drug resistance activity. After 7 days in culture, villous fragments showed a similar increase in 3H-vinblastine accumulation (143+/-10% at 180 min n=4), which was not significantly different from that in fresh tissue. Following transduction, immunohistochemistry revealed increased P-gp expression. However, the distribution of the protein differed from that in controls, with P-gp being located throughout the tissue as opposed to the normal specific location on the maternal facing plasma membrane. Transduced explants showed a significantly larger increase in 3H-vinblastine accumulation in the presence of cyclosporin A than control explants (245+/-15.5% at 180 min, n=4), suggesting reduced capacity to efflux vinblastine. This study demonstrates P-gp activity in intact placental tissue which is maintained in explant culture. Retroviral transduction of P-gp to such tissue leads to increased but undirected expression of the protein. The consequent increased activity at sites such as the basal, fetal facing, plasma membrane probably explains the increased substrate accumulation within the tissue.