SummaryThirty-six healthy volunteers, 19 men and 17 women, were selected to study the possible influence of carbamazepine on Li+ ratio in vitro. The study used within-Ss design. The procedure for preparing and calculating Li + ratio in vitro followed Pandey's method. Twenty milliliters of blood was withdrawn from each subject, and divided evenly into three test tubes containing sodium heparin. Test tube 1 (control) went through the regular Pandey's method; test tube 2 went through the same procedure, except both the loading and incubation media contained 35 μmol/L of carbamazepine. The result showed that carbamazepine did not significantly alter Li+ ratio in vitro. The neurotoxicity resulting from lithium-carbamazepine combination might be related to carbamazepine alone or preexisting physical conditions or interactions not related to Li+ ratio.