The mesonephros is the functional adult kidney in amphibia and fish and is an essential precursor for the formation of the metanephros. Thus, gene defects which prevent formation of a mesonephros inevitably result in total anephric neonates. The transient mesonephros consists, generally, of a small number of nephrons (30-70) which contain a glomerulus, proximal and distal tubules, but no loop of Henle (Moritz and Wintour 1999; Ludwig and Landman 2005; Wrobel 2001). In some species, the collecting duct drains via the cloaca and urachus, into the allantoic compartment. In those species in which the placenta is cotyledonary (sheep, cattle) or diffuse (pig), the accumulation of allantoic fluid allows the growth of the allantoic membrane to reach the whole uterine surface, including caruncles (the predetermined site of placenta formation) in both horns of the uterus. However, the allantois can also reabsorb fluid. Fetal vascularization of the placenta is carried via the allantois. In many species, there is unequivocal evidence that the mesonephros functions both as an excretory organ (sheep, pig, rabbit) and as a source of circulating hormones and enzymes (erythropoietin, renin) and contains receptors for glucocorticoids, mineralocorticoids and growth factors such as insulin and IGF-1 (Peers et al. 2001; Kitraki et al. 1997; Leeson 1959; Korgun et al. 2003) as well as Ang II receptors (Butkus et al. 1997). In addition, the mesonephros has been shown to contribute cells to the developing gonads, and the aorta-gonad-mesonephric (AGM) region is a source of haemopoietic stem cells (Sainio and Raatikainen 1999). The glucocorticoid receptors found in the sheep were shown to be functional because a 48-h exposure of the developing fetus, at a time when only mesonephros was present (26-28 days), produced a significant alteration in allantoic fluid composition (Peers et al. 2001).