Patch clamp methods were used to characterize the channels on the apical membrane of initial collecting ducts from Ambystoma tigrinum. Apical membranes were exposed by everting and perfusing fragments of the renal tubule in vitro. Tubules were dissected from two groups of animals; one maintained in tap water, and the other kept in a solution of 50 mM KCl from seven to nineteen days. Patches of apical membranes on tubules taken from animals exposed to tap water expressed low-conductance amiloride sensitive sodium channels (ENaC) in 22 of 49 patches. Only three maxi K channels were observed in this group. In animals exposed to KCl, low-conductance amiloride sensitive sodium channels, 3.7 +/- 0.2 pS (36 of 45 patches) and high-conductance 98.3 +/- 5.0 pS (19 of 45 patches) potassium channels were observed. The estimated density of apical maxi K channels increased dramatically from 0.08 to 0.76 channels/mu 2 in tubules taken from animals exposed to KCl. All but four of nineteen patches which contained maxi K channels also expressed the low conductance sodium channels. Therefore, at least 85% of the maxi K channels studied were in principal cells. We speculate that the increase in maxi K channel activity may represent a mechanism for enhancing the potassium secretory capacity of the initial collecting duct. As expected, exposure of the animals to 50 mM KCl prior to dissection of the initial collecting ducts also increased the estimated density of ENaC from 0.99 to 3.89 channels/mu 2. This upregulation of sodium channel activity is presumably related to the widely recognized effect of potassium loading to increase the plasma aldosterone level.