The pharmacokinetics of alpha-difluoromethylornithine (DFMO), an irreversible inhibitor of polyamine biosynthesis, were investigated in BALB/c (nude) mice after i.p. injection and after oral administration of radiolabeled drug. After i.p. injection the compound was rapidly cleared from the serum (t1/2 alpha = 14 min; t1/2 beta = 2.1 h) and from tissues such as muscle, liver and kidney (t1/2 alpha = 30-60 min; t1/2 beta = 2.1 h). DFMO concentrations were proportional to the administered dose (10-2000 mg/kg) in both serum and tissues. Oral administration of DFMO was carried out by dissolving the compound in drinking water at a concentration of 20 g/l. Studies on the distribution showed that DFMO did not accumulate preferentially in any particular tissue. An extremely wide variation in the dose actually achieved in different animals was observed; this ranged from 350 to 2800 mg/kg for a 14-h treatment period. A significant correlation (r = 0.83-0.92) between the dose of DFMO, calculated from the consumption of drinking water for each individual animal, and the DFMO concentrations in serum, muscle, spleen, liver and kidney was found. Similarly, it was shown that oral administration of DFMO during the daytime resulted in 10- to 15-fold lower levels than administration during the night. After discontinuation of treatment DFMO levels in serum and tissues decreased by 50% in approximately 6 h. From these results it is concluded that the optimal treatment schedule of mice with DFMO (or other drugs with similar pharmacodynamic properties) consists in a combination of oral administration via the drinking water and additional i.p. injection (during the daytime). Furthermore, the drug intake of the individual animals should be monitored to check whether the experimental requirements are actually fulfilled.