In the village population of Mali situated in the sudan savanna, malariometric indices and hematocrit values were performed before and after mass drug administration given every 15 days by a nurse to three-quarters of the population, with the remaining quarter receiving a placebo. In this population, endemic malaria was stable and high, essentially caused by Plasmodium falciparum. The "protected" population showed a decrease of parasite rate and spleen rate with concomitent rise in hematocrit values. Following the rainy season, when transmission of malaria is highest, the normally observed increase in malariometric indices with lowered hematocrit values were not seen during chemoprophylaxis. While splenomegaly found in rural children is related to malaria, the main etiology of anaemia observed in this age group is probably malaria but a role is played by others factors.