We studied the synthesis of calmodulin mRNA and of calmodulin during the asexual cell cycle of Plasmodium falciparum. The expression of the gene was low during the early stages of the erythrocytic asexual cycle. When the parasites were 28 h old the level of mRNA increased steadily to reach a peak at the age of 40 h. During the last 8 h of the cycle a constant decrease in the amount of calmodulin mRNA was observed. The synthesis of the protein followed the expression pattern of the mRNA, but without the final decrease. The average amounts of calmodulin measured were 2.5 +/- 0.9 ng/10(8) 14-h-old rings, 4.38 +/- 0.35 ng/10(8) 36-h-old trophozoites, 11.8 +/- 1.45 ng/10(8) 44-h-old trophozoites, and 19.6 +/- 1.9 ng/10(8) 48-h-old schizonts. The in vivo stability of mRNA in different developmental stages was evaluated by pulse and chase experiments. The RNA of very mature trophozoites was quickly degraded, in contrast with the high stability shown by the RNA in other stages. The results here suggest that the calmodulin gene expression in Plasmodium does not follow a housekeeping pattern of expression such as in other eukaryotic cells, but that it is regulated at the level of transcription and RNA degradation.