In the face of growing chloroquine resistance of Plasmodium falciparum, efforts to prolong the clinical usefulness of the drug have partly concentrated on its combination with potential resistance-reversing compounds. However, clinical studies on such combinations have been limited. We have compared the efficacy of halofantrine, an arylaminoalcohol effective in chloroquine resistant malaria, and a combination of chloroquine plus chlorpheniramine, a histamine H1 receptor antagonist which reverses chloroquine resistance of P. falciparum in vitro and in vivo, in 100 children with acute symptomatic uncomplicated falciparum malaria in an area in Nigeria where the rate of chloroquine resistance is 35-45%. Both chloroquine plus chlorpheniramine and halofantrine produced similar parasite and fever clearance times and cure rates (96%). Both treatment regimens were relatively well tolerated. Pruritus was commoner in patients treated with chloroquine plus chlorpheniramine than in those treated with halofantrine. Intravascular haemolysis occurred in one patient, and abdominal pain with or without diarrhoea occurred in 4 patients, treated with halofantrine. In vitro, the chloroquine resistance of P. falciparum isolates obtained from the patients was reversed by verapamil. All patients with isolates which were chloroquine-resistant in vitro were cured by either therapy. These results indicate that chloroquine plus chlorpheniramine is as effective as halofantrine and is without overt deleterious effect in treating acute uncomplicated chloroquine-resistant falciparum malaria in children, and may be a clinically useful alternative for this purpose in Nigeria.