The in vivo response of Plasmodium falciparum to standard treatment with sulphadoxine/pyrimethamine was studied in 19 hospital patients from Yekepa town with hypoendemic malaria and in 28 children, two to nine years old, living in a village with holoendemic malaria. In vitro tests were performed on eight isolates. In the hospital patients all parasites cleared with mean clearance time of 2.2 (range one to three) days and no recrudescence occurred during a 28-day follow-up period. In the village children, despite a high sporozoite inoculation rate, recurrent parasitaemias were only recorded after 28 days, suggesting a rather long-lasting prophylactic effect against reinfection by the drug combination. In vitro, inhibition of parasite multiplication was achieved by 3 X 10(-7) M sulphadoxine and 3.8 X 10(-9) M pyrimethamine.