We investigated the expression of several mRNAs in exoerythrocytic and erythrocytic stages of Plasmodium yoelii in infected mice, focusing our attention on genes thought to be involved in signal transduction (like pypka and pymap-1, encoding homologues of cAMP-dependent and mitogen-activated protein kinases, respectively) and cell cycle progression (those encoding the cdc2-related kinases Pycrk-1, Pycrk-3 and Pymrk). Messengers coding for enzymes involved in general processes such as DNA replication and RNA transcription (both subunits of the ribonucleotide reductase (Pyrnr1, Pyrnr2) and RNA polymerase II) as well as a messenger coding for Pys21, a sexual stage-specific protein, were also investigated. Total RNA was prepared from livers of infected mice at different times post sporozoite inoculation. In contrast to the pys21 transcript, which was observed only in infected erythrocytes, all messenger species could be detected in the liver by RT-PCR, peaking at 43 h post infection, a time when parasite burden was maximum, and decreasing markedly thereafter to become hardly visible at 168 h. Some transcripts (pypka, pymap-1, pyrnr1 and pyrnr2) could be detected 12 h after infection, while others (pymrk and pyrnapolII) did not become detectable until 24 h. In addition, we characterised all these messengers by Northern blot of total RNAs extracted from infected erythrocytes. Taken together, these data suggest that a similar set of regulatory genes is expressed during both exoerythrocytic and erythrocytic schizogony.