Utilizing a general stochastic theory of drug kinetics we have demonstrated that from the serum concentration curve derived from a single dose of lithium, it is possible to predict the steady state concentration, the time required to reach steady state, the dependency of these quantities on the time interval between drug administrations and the fluctuations in plasma level under steady state conditions.2. It has been shown in general that the more frequently the drug is administered the higher is the steady state level, the shorter is the time required to reach steady state, and the smaller are the fluctuations.