A simple method for short-term in vitro cultivation of malaria parasites in nonnucleated erythrocytes permitted measurements of DNA, RNA, and protein synthesis throughout one intra-erythrocytie schizogonic cycle of Plasmodium knowlesi. Erythrocytes parasitized with ring forms of P. knowlesi were bled from infected rhesus monkeys and incubated in vitro until they become schizonts. The addition of either (6-14C) orotic acid or (8-14C) adenine to cultures showed a slow rate of incorporation of these radionuclides into DNA during the ring and trophozoite stage of parasitic growth with exponential utilization of these radioisotopes during the phase of nuclear development and division. RNA synthesis occurred at a rate that seemed intermediate between linear and exponential. Protein synthesis was measured by the incorporation of radiolabeled amino acids into infected cell cultures and occurred at a linear rate. This unique association of exponential synthesis of DNA and linear protein synthesis seemed to be characteristic of plasmodial development. These studies of plasmodial growth illustrate the usefulness of radioisotopes to study the growth and metabolism of malaria parasites and provide a method for investigating the pharmacologic action of antimalarial drugs.