• In previous research, evidence was provided of genetic control of in vitro red blood cell (RBC) uptake of lithium ion following a 24-hour incubation using a monozygotic (MZ)-dizygotic (DZ) twin study method. To extend these data, in vitro RBC lithium ion concentrations were assessed for five MZ and five DZ twin pairs. To extend this line of research on genetic control of lithium ion distribution to in vivo conditions, these twin pairs were administered lithium carbonate for seven days during which time RBC and plasma lithium ion concentrations were assessed. In this limited sample, the data provide evidence of genetic control of distribution of lithium ion across the RBC membrane in vivo as well as of in vitro RBC uptake of lithium ion.