A comparison is made between atebrin-musonate and quinine bihydrochloride in the treatment of acute malaria. 286 cases of acute malaria due to Malayan strains of P. falciparum, P. vivax, and P. malariae, were treated in alternating sequence with one or other of these drugs. The rates at which the atebrin-musonate and the quinine case groups became trophozoite-free and fever-free are contrasted in a series of graphs. It is shown that there was a tendency for trophozoites to disappear from the peripheral blood and for temperatures to fall to normal somewhat earlier among cases treated with atebrin-musonate. No toxic effects of any importance were observed (but see footnote p. 657). Evidence is recorded which suggests that the minimal effective daily dose for an adult is 0·375 gramme (= atebrin 0·3 gramme). This dose when given either intramuscularly or intravenously on two successive days effected a rapid disappearance of parasites and fever. Intramuscular administration is regarded as the method of choice. It is noted that absorption of the drug from the muscles is very rapid, and that atebrin may be demonstrated in the urine within 10 minutes of an intramuscular injection of 0·3 gramme. A method of testing for the presence of atebrin in the urine which is sensitive to over one in a million is described. It was not possible to obtain precise data regarding the permanency of cure but an analysis of cases returning to hospital within 10 weeks of discharge suggests that relapses after atebrin-musonate treatment are probably fairly common.