Angiotensin-converting enzyme (ACE) cleaves Angiotensin I (Ang I) to form Angiotensin II (Ang II). Angiotensin-converting enzyme 2 (ACE2) is a recently described ACE homolog [65] . Both ACE and ACE2 are zinc-dependent peptidases of the M2-metalloprotease family. Within the renin-angiotensin system (RAS), ACE2 competes with ACE because it is capable of hydrolyzing Ang I into the nonapeptide Ang(1-9) [65] . In humans, ace2 gene expression was predominantly detected by Northern blot analysis in kidney, heart and testis [65, 66] . In addition, a moderate expression was also observed in several other tissues including the ovary [66] . Using semiquantitative RT PCR, a wide distribution was observed in rat tissues [67] . In mammals, previous observations suggested that the renin-angiotensin system was functional in the ovary. In cattle, a greater expression of Ang II was observed in large follicles. In addition, several lines of evidence supported the idea of Ang II in blocking the inhibitory effect of theca cells on meiosis resumption of bovine oocytes [68] . In brook trout (Salvelinus fontinalis) salmon Ang I and human Ang II were both able to increase the level of in vitro spontaneous ovulation [69] . In the present transcriptomic study, we observed a dramatic increase of ace2 gene expression during the preovulatory period. This observation was confirmed by real-time PCR data. Together, these observations suggest that the dramatic upregulation of ace2 gene immediately prior to ovulation is important for the ovulatory process. In mammals, little is know about the role of ACE2 in the ovary. However, it is known in mammals that ACE2 can function as an Ang II degrading enzyme, forming the vasodilatator peptide Ang(1-7) [70, 71] . Interestingly, a local vasodilatation is a key characteristics of the inflammatory response that is also observed during the mammalian ovulatory process (see [50] for review). Therefore, it can be hypothesized that the observed increase of ace2 gene expression in the trout preovulatory participates in the vascular dynamics changes that are putatively occurring during the ovulatory process.