Transplantation of embryonic kidneys has been proposed as a potential solution to the problem of kidney donor shortage. We have reported previously that rat metanephroi transplanted into a host of the same species have glomerular filtration rates comparable with those achieved by dialysis [1]. Here we estimate nephron number and examine the expression of key transporters central to the concentration of urine in order to determine the maturity of transplanted tissue. Metanephroi from embryonic day 15 (E15) Lewis rat embryos were transplanted adjacent to the abdominal aorta of adult female Lewis rats under isoflurane anaesthesia (1 l/min O₂, 2.5% isoflurane). Post-operative checks on these rats were made following recovery from anaesthesia. Metanephroi were explanted between 21 days and 3 months later and fixed in 4% paraformaldehyde or snap frozen. Glomeruli were counted in transplanted metanephroi at varying ages post-transplant and compared with embryonic day 21 (E21) and postnatal day 1 (PN1) Lewis rat kidneys using a non-biased stereological counting technique involving counting glomeruli within paired serial sections of kidneys using a modified version of the dissector method [1]. Aquaporin 1 and 2 (AQP 1 and 2) and urea transporters A-1,2 and 3 (UT-A1,2 and 3) were localised by immunohistochemistry. Relative gene expression of these transporters was carried out by quantitative PCR (qPCR). Statistical analysis was by one-way ANOVA; all values are expressed as mean ± S.E.M. Number of glomeruli in transplanted metanephroi (4399 ± 216, n = 9) were significantly lower compared to PN1 kidneys (7023 ± 587, n = 5, P < 0.05) but not E21 kidneys (2578 ± 358, n = 5, P > 0.05). Most metanephroi expressed AQP1 (5/6), AQP2 (4/5) but not UT-A1,2 or 3. This pattern of expression was also observed in E21 and PN1 Lewis rat kidneys. qPCR compared levels of gene expression in transplanted metanephroi, E21 and PN1 samples to adult kidney tissue (given an arbitrary expression of 1). AQP 1 gene expression in transplanted metanephroi (0.10 ± 0.02, n = 5) was not significantly different compared to either E21 (0.20 ± 0.08, n = 6, P > 0.05) or PN1 kidneys (0.30 ± 0.12, n = 7, P > 0.05). AQP2 gene expression in transplanted metanephroi (0.10 ± 0.04, n = 6) was significantly lower compared to both E21 (0.24 ± 0.04, n = 7, P < 0.001) and PN1 kidneys (0.24 ± 0.05, n = 7, P < 0.001). Gene expression for UT-A1/2 was very low in transplanted metanephroi (0.03 ± 0.01, n = 6) but not significantly different from E21 (0.03 ± 0.01, n= 7, P > 0.05) or PN1 kidneys (0.10 ± 0.03, n = 7, P > 0.05). UT-A1/3 gene expression was undetectable in transplanted metanephroi (n = 6) but E21 kidneys (0.02 ± 0.00, n = 7) had significantly lower expression than PN1 kidneys (0.04 ± 0.01, n = 7, P < 0.05). These results provide evidence that transplanted metanephroi, up to 3 months following transplantation, are at a stage of nephrogenesis comparable with kidneys at around the time of birth in the rat.