The clinical and parasitologic efficacies of oral chloroquine phosphate, pyrimethamine/sulphadoxine and pyrimethamine/sulphalene in treating Plasmodium falciparum malaria were assessed in selected sites of northeastern Nigeria (Zone D of the Primary Health Care (PHC) Programme) using a 14-day standard in-vivo protocol during 1988-1990. Of a total of 2056 children under 5 years screened for infection, for chloroquine trials, 1189 (57.8%) were positive for Plasmodium infection. One hundred and seventy (14.3%) of these positive children were enrolled into the study. Clinically, the drug demonstrated high performance in clearing symptoms of infection. However, varying degrees of parasitologic failure, ranging from delayed clearance through recrudescence to asymptomatic Type-II resistance, were encountered. For tests with pyrimethamine/sulphadoxine and pyrimethamine/sulphalene, 517 and 253 children, respectively, were screened. The corresponding infection rates were 71.6% (370 children) and 71.5% (181 children), with 59 and 34 enrollments. Both drugs were highly effective, clinically and parasitologically. These findings and their implications for the success of the PHC programme for malaria control are discussed.