The pharmacokinetics of a series of heterocyclic compounds substituted with amidino or imidazolinyl groups and showing trypanocidal and antileukemic activity has been studied in mice and rats using radiolabelled material and newly developed HPLC techniques. Trypanocidal compound 261/115 (2-Amidino-indole-6-carboxamidine) showed species differences in mice and rats, however, in mice no differences were detected after i.p. and s.c. administration. Terminal half-life was 2.4 h and 5 days in mice and rats, respectively. Trypanocidal compound 102/198 [2-(4-Amidinophenyl)indole-6-carboxamidine), DAPI) showed a terminal half-life of 60 days in rats. For both compounds excretion data were determined, accounting for less than 1% in bile in both cases, 36% and 7.5% in urine and 36% and 22.5% in faeces for 261/115 and 102/198, respectively. According to an organ balance and a computer fit of excretion data extensive tissue binding seems to be responsible for this long terminal half-life. This is also suggested by RBC/plasma partitioning data. Since biliary excretion is negligible, direct excretion into the gut might be responsible for the excretion in faeces. For other antileukemic diamidines as well as for analogous diimidazolinyl compounds consisting of two heterocyclic nuclei linked by a double bond plasma pharmacokinetics were determined in mice after i.p. administration. Their terminal half-lives ranged between 3.5 and 10.7 h. For all compounds studied multi-compartmental models could be established. In addition, it could be shown for 261/115 and 150/129 [E)-2.2'-Vinylenedi-1-benzofurane-5-carboxamidine) that their pharmacokinetics were not dose-dependent. In general, imidazolinyl compounds showed higher plasma levels than did their amidine analogues. This property seems to be related to their solubility. However, no correlation could be found between pharmacokinetic properties and antileukemic activity. Therefore other properties like DNA-binding, tissue distribution etc. should be considered in evaluating new strategies for the development of antileukemic compounds.