Relatively little information is available concerning the expression of parasite genes during the liver stage of Plasmodium infection, mostly because of low-level infection of host hepatocytes and the lack of purification techniques for the liver stage parasites. We have determined the optimal dosage of Plasmodium yoelii sporozoite inoculum and routes of inoculation, which are intravenous tail vein and the intrahepatic portal circulation. To determine which route was optimal, BALB/c mice were inoculated via 1 of these routes, and parasitemia was detected by reverse transcription polymerase chain reaction (RT-PCR) detecting both murine β-actin and P. yoelii-specific 28S ribosomal RNA in the liver samples. Murine β-actin was detected after 15 cycles of PCR, and its expression levels did not differ between treatment groups. However, P. yoelii-specific 28S ribosomal RNA gene product was detected after 15 cycles of PCR in animals inoculated via the tail vein but was not detected until 25 cycles in animals inoculated via the intrahepatic portal circulation. Experiments were then performed to identify the smallest inoculum required to initiate a liver stage infection that would yield sufficient parasite RNA for analysis. Inoculation with different doses of sporozoite inocula was followed by RT-PCR on the livers of the inoculated animals. The P. yoelii-specific 28S ribosomal RNA gene product was first detected in both treatment groups after 15 cycles, suggesting that both doses of sporozoite inocula provided relatively the same level of liver infection rate. We also have analyzed infected mouse liver for parasite-specific mRNA, which was detectable as early as 24 hr postinfection.