To examine whether the functional and morphologic conversion of the neonatal ascending thin limb (ATL) of Henle's loop is related to gestational length, we evaluated the transepithelial voltages (Vts) of ATLs in perinatal mouse, hamster, rabbit, and rat kidneys. In isolated microperfused tubule preparations, Vts of neonatal ATLs were 23.8 ± 1.4 in mouse, 25.7 ± 2.2 in hamster, and 18.2 ± 1.6 mV in rabbit. The influence of gestational length on the Vts and rat Na-K-Cl cotransporter (rNKCC2) expression pattern was also examined in perinatal rats subjected to a prolonged gestation due to either a daily s.c. injection of 5 mg progesterone or ligation of the extremities of the uterine horn. Vts of d 3 neonates were 2.9 ± 1.0 (p < 0.0001 versus d 0); Vts of d 23 fetuses subjected to ligation were 4.9 ± 0.8 (p < 0.005 versus d 0); and Vts of d 23 fetuses given progesterone were 3.4 ± 1.7 mV (p < 0.001 versus d 0). rNKCC2 expression tended to disappear in the renal papillae of d 23 fetuses. Our data demonstrate that the perinatal conversion of the ATL is a phenomenon commonly observed among rodents; furthermore, it is dependent on the gestational length, but unrelated to the birth process.