BACKGROUND: Until recently, the corpus luteum (CL) was considered to be the main source of progesterone (P4) during pregnancy in the domestic cat (Felis catus). However, other possible sources of P4 have not been ruled out. Although feline placental homogenates were found to be capable of synthesizing P4, expression of the respective steroidogenic enzymes has not been investigated at the molecular level. Therefore, in the present study, expression of the two major factors involved in the synthesis of P4 - 3beta-hydroxysteroid dehydrogenase (3betaHSD) and steroidogenic acute regulatory protein (StAR) - was investigated in the feline CL and placenta during the course of pseudopregnancy and pregnancy. METHODS: The mRNA levels of StAR and 3betaHSD were determined using Real Time PCR and their localizations were determined by immunohistochemistry. Placental P4 concentrations, after ethyl extraction, were measured by EIA. RESULTS: Luteal 3betaHSD and StAR mRNA levels were strongly time-dependent, peaking during mid-pregnancy. The placental 3betaHSD mRNA level was significantly upregulated towards the end of pregnancy. In the CL, 3betaHSD and StAR protein were localized in the luteal cells whereas in the placenta they were localized to the maternal decidual cells. Placental P4 concentrations were low in early pregnant queens, but increased along with gestational age. CONCLUSIONS: These results confirm that the placenta is an additional source of P4 in pregnant queens and can thereby be considered as an important endocrine organ supporting feline pregnancy.