We are able to evert and perfuse rat cortical collecting tubules (CCT) at 37 degrees C. Patch-clamp techniques were used to study high-conductance potassium channels (maxi K) on the apical membrane. Under control conditions (150 mM Na+ and 5 mM K+ in pipette and bathing solutions), the slope conductance averaged 109.8 +/- 6.6 pS (12 channels), and reversal potential (expressed as pipette voltage) was +26.3 +/- 2.4 mV. The percent of time the channel spends in the open state and unitary current when voltage was clamped to 0 mV were 1.4 +/- 0.7% and 3.12 +/- 0.42 pA, respectively. In six patches voltage clamped to 0 mV, the isosmotic solution perfused through the everted tubule (basolateral surface) was exchanged for one made 70 mosmol/kgH2O hyposmotic to the control saline. Open probability increased from 0.019 to 0.258, an increase of 0.239 +/- 0.065 (P < 0.005). In four patches where a maxi K channel was evident, no increase in open probability was observed when a hyposmotic saline was placed on the apical surface. However, when vasopressin was present on the basolateral surface, apical application of hyposmotic saline resulted in a series of bursts of channel activity. The average increase in open probability during bursts was (0.055 +/- 0.017, P < 0.005). We conclude that one function of the maxi K channel located in the apical membrane of the rat CCT may be to release intracellular solute (potassium) during a volume regulatory decrease induced by placing a dilute solution on the basolateral surface or when the apical osmolarity is reduced in the presence of vasopressin. These data are consistent with the hypothesis that the physiological role of the channel is to regulate cell volume during water reabsorption.