Medullary thyroid carcinoma (MTC) is frequently resistant to chemotherapy. Multidrug resistance (MDR) is one of the involved mechanisms. In this work we have studied the MDR1 gene expression in five MTC human cell lines that we have isolated and we have compared this expression to that of normal thyroid tissue. We have also tried to reverse the resistance to doxorubicin with verapamil (VRP) and ciclosporin A (CSA). MDR1 ARNm expression was studied and quantified by polymerase chain reaction (PCR) in normal and pathological thyroid tissues. The doxorubicin-induced cytotoxicity was evaluated with the 3,-4,5 dimethylthiazol-2,5 diphenyl tetrazolium bromide (MTT) test, the neutral red (NR) uptake and with total glutathione (GSH) or intracellular lactate dehydrogenase (LDH) measurements. We found an increase of MDR1 ARNm in MTC as compared with normal tissues. Doxorubicin was cytotoxic after a 48-h coincubation with the cells. Three microM CSA and 10 microM VRP reversed the doxorubicin resistance only after a 48-h coincubation, generally followed with a 24 h-post-incubation. In these conditions, the GSH levels were decreased only by VRP in all the five cell lines. In conclusion, a chemoresistance related to the MDR1 gene overexpression was found in the five human MTC lines tested. VRP and CSA reversed the resistance to doxorubicin in all the MTC cell lines tested.