The asexual, intraerythrocytic stage of Plasmodium chabaudi develops synchronously in CBA mice. This in vivo synchrony has been exploited in in vitro pulse-labelling experiments to investigate the stage specificity of macromolecular synthesis by malaria parasites. Groups of mice were infected on day 0 with P. chabaudi and on day 3, individual mice were killed at three hour intervals, and the parasitised blood labelled in vitro for 2 h with radioactive precursors of protein or nucleic acid synthesis. By taking 11 samples covering one and one-third parasite division cycles, it was shown that the synthesis of many parasite polypeptides was restricted to defined morphological stages of parasite development. Other polypeptides were synthesised more or less continuously during the growth cycle. The synthesis of at least 6 polypeptides was confined to schizont or merozoite differentiation. RNA synthesis was shown to increase in rate steadily during parasite growth and to fall sharply during merozoite invasion. Approximately 40% of DNA synthesis was shown to occur during trophozoite growth, but the majority (60%) was confined to a short 4-6 h period at or just before schizogony.