A modification of the standard World Health Organisation 7--day in vivo test was used to assess the parasitologic and, to limited extent, the clinical response of children less than 5 years of age to defined oral dosages of chloroquine and pyrimethamine-sulphadoxine during the 14 days following the initiation of treatment. The study took place in Jato-Aka, a rural community in Benue State of Nigeria. 471 children were screened and 271 (59%) of these had plasmodium parasites thus showing a high transmission rate at a time of the year with scarce rainfall. Of the 42 children on chloroquine and who were followed up to day 2, 4 or 9.5% of them were parasitologic failures while none of the 45 children on pyrimethamine-sulphadoxine (P-S) failed parasitologically. By day 7, 21 (50%) of the children on chloroquine and 4 or 8.9% of those on P-S. had become parasitologic failures. The total number of parasitologic failures on day 14 were 21 (50%) and 4(8.9%) for chloroquine and P-S groups respectively. Chloroquine improved the clinical state of the patients better than P-S. However, P-S appears superior in clearing the parasites. These results confirm the existence of both chloroquine and pyrimethamine-sulphadoxine resistant strains of P. falciparum in some regions of Nigeria. The routine chloroquine chemoprophylaxis of children under 5 years of age should be discontinued so as not to hasten the intensification of chloroquine resistance and because of its probable marginal efficacy at chemoprophylaxis dosages. There is also need for a national policy on pyrimethamine-sulphadoxine usage in order not to hasten its resistance to P. falciparum.