Subtherapeutic doses of chloroquine (CQ) are considered to promote development of Plasmodium falciparum resistance but little is actually known about the drug levels in the population in endemic areas. We have therefore measured blood concentrations of CQ in Tanzanian schoolchildren and related these to parasite microscopy. A total of 163 children (median age 11 years) in a suburb outside Dar es Salaam were followed during four weeks. Thick and thin blood films were obtained once weekly. Parasites were counted in 200 visual fields. CQ and desethyl-chloroquine (DECQ) were determined with HPLC in 100 microliters of capillary blood. During the study P. falciparum trophozoites were detected in a mean of 78% of the children, P. falciparum gametocytes in 7.7% and P. malariae parasites in a mean of 13%. The cumulative prevalence of P. falciparum trophozoites and P. malariae parasites was 96% and 28% respectively. On day 0 and day 28, CQ was found in 78% and 80% of the children and DECQ in 21% and 31% of them. A total of 19% of all children had a verified CQ intake during the study and 35% had probably taken CQ. With a few exceptions (9% had CQ concentrations > 100 nmol/l) drug levels were not sufficient to affect parasites with a reduced CQ susceptibility but could possibly promote development of resistance by eradicating the most susceptibility part of the parasite population.