Developmental expression of ion channels possibly participating in regulatory volume decrease was studied in rat embryonic (day E17) and perinatal (days P1-6) ureteric bud and in postnatal (P9-14) cortical collecting duct cells in primary monolayer culture. In isotonic bath solution, whole cell conductance (in nS/10 pF) was highest in E17 (4.0 +/- 0.5, n = 31) compared with P1-6 (2.0 +/- 0.1, n = 16) and P9-14 (1.3 +/- 0.2, n = 12) due to a decreasing contribution of a DIDS-sensitive Cl conductance, from E17 (2.8 +/- 0. 7, n = 12) to P1-6 (0.53 +/- 0.07, n = 9) and P9-14 (0.05 +/- 0.1, n = 7). Cl conductance in E17 exhibited a permselectivity of I approximately Cl approximately Br >> gluconate, and it activated time dependently. Hypotonic bath solution induced a large increase of whole cell conductance in P1-6 and in P9-14 but not in E17 (by 20. 0 +/- 3.7, 21.5 +/- 5.5, and 4.9 +/- 1.7; n = 11, 12, and 25, respectively) due to the activation of a time-dependently inactivating Cl conductance with a permselectivity of I >/= Br > Cl >> gluconate. In conclusion, the expression of Cl channels, as studied in vitro, appears to shift from an apparently constitutively active embryonic to a hypotonic swelling-activated type during late embryonic development of the collecting duct.