• Mathematical simulations and nonlinear numerical methods for characterizing the serum elimination half-life, volume of distribution, and renal clearance of lithium after multiple oral doses of lithium carbonate and two or more blood level determinations were constructed and adapted for clinical use. These characteristics were measured for 24 inpatients; predictions were made with the simulation and compared with actual succeeding blood levels. The average elimination half-life of 25.9 hours was compared with half-lives reported after single oral doses. Average and median errors of prediction were negligible; the root mean square error was 0.12 mEq/L, near the coefficient of variation of 0.082 mEq/L. The correlation between predicted and observed levels (r) was.82; a small systematic effect of weight was found.