Although many previous investigations have focused on in vitro studies of lithium transport by erythrocytes (RBCs) of psychiatric patients, the extent to which such studies actually reflect the transport of this drug by other types of cells in vivo is unknown. To study lithium transport in vivo, pharmacokinetic analysis of plasma lithium concentration data was performed in four subjects who were given single oral doses of lithium carbonate (600 mg). The data were analyzed according to a two-compartment model, consisting of a central compartment (extracellular, including plasma) and a peripheral (intracellular) compartment. Rate constants for the transfer of lithium into (ki) and out of (ko) the intracellular compartment were calculated. In RBCs from the same subjects, lithium transport in vitro was also directly measured. Rate constants were determined for phloretin-sensitive transport (ka), which corresponds to Na+-Li+countertransport activity, and residual passive “leak‘’ diffusion (kr). In RBCs, these two pathways account for major components of lithium efflux and influx, respectively. To compare the in vivo and in vitro rate constant data, the ratiosko/kiandaa/krwere also calculated. There was a significant correlation between these two rate-constant ratios (r= 0.96,p< 0.05), although the values observed in vitro were higher than those found in vivo. Because the in vivo rate constants reflect lithium transport by many types of cells in the peripheral compartment, this finding supports the idea that the RBC may provide a useful model for studying lithium transport processes that are also operative in other types of cells. In addition, these findings suggest that lithium pharmacokinetics may be affected by clinical conditions that alter cellular lithium transport.