The kinetics of lithium in the serum, liver and brain of rats is described. The serum levels resembled those of man, whereas considerable quantitative differences were observed when comparing specific kinetic parameters. The brain level increased with the increasing doses, approaching the corresponding serum level. Concentration differences between different brain areas could be observed only after repeated administrations. Striatum, cortex and hippocampus showed significantly higher levels than the thalamus. The liver content remained low with increasing doses, and was below the brain level.