The rat tail artery enriched by incubation in a lithium-substituted medium loses Li at three distinct rates upon return to a normal medium at 37 °C. The first is very rapid with a single exponential rate constant of the order of 1 min<sup>–1</sup>, while the second and third are considerably slower with rate constants of the order of 3 h<sup>–1</sup> and 0.6 h<sup>–1</sup>, respectively. Both slow components are cellular since they combine into a single component when active Na transport is inhibited by the absence of external K, by ouabain, or by low temperature. None of these procedures affects the rapid loss of Li from extracellular locations. It follows that cell Li can be measured simply by washing out extracellular Li at 2 °C, a procedure which removes only a small definable amount from cells. The influx of Li into cells during incubation in Li-substituted medium follows 1st order kinetics and exchanges 1:1 with cell K after a delay of about 20 min at 37 °C. This delay in influx is associated with active Na transport since it is abolished by the absence of external K, by low temperature, and by ouabain. The activation energy required for the influx of Li and of Na in K-free media is consistent, in the main, with passive diffusion through a restricted area although there is a suggestion in the case of Na that an additional mechanism (e.g. carrier) may be involved.