Physiological studies have shown that the Na+-H+ exchanger (NHE) is a major carrier protein regulating the intracellular pH in the cells of the cochlea. The presence of multiple forms of the exchanger has been demonstrated by the recent cloning of four mammalian NHEs, NHE-1 to NHE-4. Despite the structural similarity, these NHE isoforms differ in their tissue distribution, kinetic characteristics, and responses to external stimuli. The present study was undertaken to examine the expression and distribution of four NHE isoforms in the guinea pig cochlea. We used reverse transcription-polymerase chain reaction to assess the expression of NHE-1-4 isoforms and non-radioactive in situ hybridization to examine their localization. Although NHE-2, -3 and -4 isoform mRNAs could be detected in the cochlear tissue, the NHE-1 message was predominant. Cloned guinea pig NHE-1-4 partial cDNA fragments were highly homologous to the corresponding rat NHE isoforms. NHE-1 isoform mRNA was distributed in the hair cells, marginal cells, spiral ligament fibrocytes, spiral prominence cells and spiral ganglion cells. NHE- localized in a variety of cochlear cells would contribute to their differential function.