Mammalian high conductance, calcium-activated potassium (maxi-K) channels are composed of two dissimilar subunits, a and p. We have examined the functional contribution of the p subunit to the properties of maxi-K channels expressed heterologously in Xenopus oocytes. Channels from oocytes injected with cRNAs encoding both a and I~ subunits were much more sensitive to activation by voltage and calcium than channels composed of the a subunit alone, while expression levels, single-channel conductance, and ionic selectivity appeared unaffected. Channels from oocytes expressing both subunits were sensitive to DHS-I, a potent agonist of native maxi-K channels, whereas channels composed of the a subunit alone were insensitive. Thus, a and I~ subunits together contribute to the functional properties of expressed maxi-K channels. Regulation of coassembly might contribute to the functional diversity noted among members of this family of potassium channels.