The antimalarial activities of crude extracts and 17 fractions from the partition of 80%-methanolic extracts of three plants (the stem bark of Croton mubango, the stem bark of Nauclea pobeguinii and the leaves of Pyrenacantha staudtii) used as antimalarial remedies in the Democratic Republic of Congo were studied both in vitro (against Plasmodium falciparum) and in mice infected with Pl. berghei berghei. The toxic effects of dried aqueous extracts of the plants were also investigated, in uninfected mice. The most active crude extracts in vitro, with median inhibitory concentrations (IC(50)) of <1 microg/ml, were found to be the methanolic and dichloromethane extracts of C. mubango, and the dichloromethane extracts of N. pobeguinii and Py. staudtii. The aqueous extract with the most antimalarial activity in vitro was that of C. mubango (IC(50) = 3.2 microg/ml), followed by that of N. probeguinii (IC(50) = 5.3 microg/ml) and then that of Py. staudii (IC(50) = 15.2 microg/ml). Results from the in-vivo tests of antimalarial activity showed that, at a daily oral dose of 200 mg/kg, all the dichloromethane extracts, the petroleum-ether, chloroformic, ethyl-acetate and residual water-soluble fractions from C. mubango, and the chloroformic, ethyl-acetate and n-butanolic fractions from Py. staudtii produced >80% chemosuppression of the parasitaemias by day 4. The aqueous extracts of C. mubango and N. probeguinii produced a slightly lower but still significant inhibition of parasitaemia (60%-80%) whereas that of Py. staudtii only suppressed the day-4 parasitaemias by 37%. The dried aqueous extract of the stem bark of C. mubango showed some signs of toxicity in mice, with median lethal doses (LD(50)) of 350 mg/kg in the female mice and 900 mg/kg in the male. The extract significantly increased the serum concentrations of glutamate-oxaloacetate transaminase (GOT) and glutamate-pyruvate transaminase (GPT) in mice of both sexes, but had no effect on the blood levels of creatinine or urea. No significant toxic effect was observed for the dried aqueous extracts of N. pobeguinii and Py. staudtii (LD(50) >5 g/kg). Neither of these extracts affected the serum concentrations of GPT or the blood concentrations of creatinine and urea, although the N. pobeguinii extract did increase the serum concentration of GOT.