The plasma lithium levels of 18 subjects receiving one standard and two slow-release preparations of lithium carbonate were measured at frequent intervals during the 24 hours following oral ingestion of a single dose of the drug. Although the slow-release tablets showed slow-release in vitro, this was not so in vivo. One slow-release preparation, in particular, was ineffectively absorbed by some subjects. There was no difference in the rate of absorption and excretion between the other slow-release product and the standard BP preparation. The implications of the results are discussed.