We studied the mechanisms of K(+) transport in cells from isolated and perfused collecting tubules and ducts from the mesonephric kidney of the toad Bufo bufo. Cells were impaled with microelectrodes across the basal cell membrane. The basolateral membrane potential (V(bl)) depolarized upon change of bath [K(+)] from 3 to 20 mmol l(-1) demonstrating a large K(+) conductance in this membrane. In collecting tubules and collecting ducts a V(bl) of -66+/-2 mV and -74+/-4 mV depolarized by 30+/-2 mV and 36+/-3 mV, respectively (N=23; 15). The K(+) channel inhibitor Ba(2+) (1 mmol l(-1)) inhibited the basolateral K(+) conductance and depolarized a V(bl) of -64+/-4 mV by 30+/-6 mV (N=8). Luminal K(+) steps (3 to 20 mmol l(-1)) demonstrated a K(+) conductance in the apical cell membrane. In collecting tubules and collecting ducts a V(bl) of -70+/-3 mV and -73+/-3 mV depolarized by 11+/-3 mV and 16+/-3 mV, respectively (N=11; 11). This conductance could also be inhibited by Ba(2+), which depolarized a V(bl) of -71+/-5 mV by 9+/-3 mV (N=5). The pump inhibitor ouabain (1 mmol l(-1)) depolarized V(bl), but addition of furosemide to bath solution did not affect V(bl). The [K(+)] in urine varied from 1.3 to 22.8 mmol l(-1). In conclusion, we propose that the collecting duct system of B. bufo secretes K(+) into the urine via luminal K(+) channels.