The pharmacokinetics of swainsonine (SW) was investigated in mice after intravenous administration of 3 micrograms/ml. The time course of SW blood levels followed a three-compartment open pharmacokinetic model which consisted of biphasic distribution, and a rapid elimination phase (terminal half-life, 31.6 min). After completion of the distribution, SW was widely distributed to the extravascular space (Vss, 22ml; Vd, 33ml). Free fractions of this substance were indistinguishable from unity, indicating little or no protein binding. The rate-limiting step in the elimination of SW from the body appears to be the slow return from the deep compartment into the central one. Accordingly, SW blood levels may be low and yet significant amounts of this agent may be present in different body organs and tissues. A comparison of SW tissue levels indicates that the highest amounts appeared in the bladder, kidney, and thymus, (3.8 0.5, and 2.2 nmoles/g wet wt) with the lowest levels consistently appearing in the brain (< 0.1 nmoles/g wet wt). Hence, this study suggest that: 1) SW has high affinity for the thymus, which is in part consistent with its previously published immunomodulatory action; 2) SW should be infused for at least 2 1/2 hrs for its concentration to approach a plateau (this is based on the short half-life of SW and its time to steady state); and 3) CNS toxicity may be dose-limiting and not be present at SW levels preventing metastasis.