Male Sprague Dawley rats (n = 6-8 per group) weighing from 300-450 g were used for the study. Each rat received a single dose of cladribine (CdA) by ia (1 mg/kg) or s.c. (2 mg/kg) injection. Pharmacokinetic data were calculated by standard procedures assuming a 2-compartment open model following i.v. bolus using WinNonLin and Rstrips, and differences between the two modes of injections were considered significance when p < 0.05. The results showed that plasma concentrations of CdA decreased rapidly following a biphasic decline after both ia and s.c. administrations. The AUC and t1/2 beta after a single 1 mg/kg ia and 2 mg/kg s.c. injection of CdA were 0.66 +/- 0.34 vs 1.2 +/- 0.3 microg x h/ml and 3.5 +/- 2.1 vs 4.5 +/- 2.2 h, respectively (p > 0.05). The mean absolute bioavailability following the s.c. injection was close to 90%. The inter-subject variability of plasma concentrations of CdA was 35% and 150% following sc and ia injections, respectively. It is concluded that the rat is a reasonably good animal model to study the pharmacokinetics of CdA in plasma, and that sc injection may produce more favourable pharmacokinetic profiles than ia injection following a single dose.