In countries of the Sahel and sub-Sahel, malaria transmission is highly seasonal. Nevertheless, mortality and morbidity from malaria may be very high during the few months of the year when malaria transmission takes place, especially in children. It has been shown previously in The Gambia and elsewhere that in areas of seasonal malaria transmission, mortality and morbidity from malaria in children can be reduced substantially by the regular administration of anti-malarial drugs during the period of risk (chemoprophylaxis). However, chemoprophylaxis is difficult to sustain. Intermittent preventative treatment (IPT) is an adapted form of chemoprophylaxis in which anti-malarial drugs are given at fixed but less frequent intervals than with chemoprophylaxis, allowing blood concentrations of anti-malarial drugs to fall below the inhibitory concentration for periods between drug administrations. This approach to malaria control was used first in pregnant women among whom it is highly effective. However, more recently, it has also been used in infants (IPTi) with anti-malarials being given at the same time as vaccines are administered. This approach is likely to be most effective in areas of high transmission where a high proportion of severe cases of malaria occur in children during the first year of life. IPTi is likely to be less effective in reducing the burden of malaria in areas of seasonal malaria transmission where many cases of severe malaria occur in older children. Intermittent preventative treatment in children (IPTc) is a possible approach to the control of malaria in areas where the main burden of malaria is in older children. Thus, the investigators have undertaken a study to determine how effective this approach would be in preventing malaria in Senegal.