Inward-rectifier, G-protein-regulated and ATP-dependent K+ channels form a novel gene family of related proteins which share two transmembrane segments as a common structural feature. These K+ channels are only distantly related to the voltage-gated Shaker-type K+ channels comprising six transmembrane segments. Although the quaternary structure of voltage-gated K+ channels has been extensively studied in the past, little is known about subunit assembly of inward-rectifier K+ channels. Differential sensitivity of inward-rectifier K+ channels to voltage-dependent pore block by spermine was used to analyse subunit assembly. It is shown that inward-rectifier K+ channel proteins are composed of four subunits whose assembly obeys the rules of a binomial distribution. 'Strong' and 'mild' inward-rectifier K+ channel subunits (BIR10 and ROMK1) which are co-expressed in individual auditory hair cells form hetero-tetramers. Distribution of these hetero-tetramers, however, is not binomial. Hetero- and homo-oligomeric channels form with similar probabilities resulting in independent channel populations with distinct functional properties.