OBJECTIVE To compare the relative expression levels of NPR-A mRNA in the stria vascularis (StV), nonstrial tissue of the cochlear lateral wall (NSt) and vestibula in the mouse inner ear to determine the potential contribution of ANP signaling in different parts to inner ear fluid homeostasis. METHODS Atrial natriuretic peptide (ANP) is a cardiac hormone known to be involved in the regulation of body fluid homeostasis. It is assumed that ANP might also participate in the regulation of inner ear fluid dynamics. ANP selectively binds with atrial natriuretic peptide receptor (NPR-A) to exert its physiological function. We have previously shown the presence of NPR-A transcripts in the mouse stria vascularis as well as in the nonstrial tissue of the cochlear lateral wall and vestibular organ by polymerase-chain reaction. METHODS Total RNA of samples of stria vascularis, nonstrial tissue of the cochlear lateral wall and vestibular organ tissue from ears of 10 adult mice was isolated, amplified by the real-time quantitative reverse transcription-polymerase chain reaction (RT-PCR) using consensus primers flanking a region of 127 bp at the target sequences. The brain of mice known to contain NPR-A was used as a positive control. The one-way analysis of variance, ANOVA and Student-Newman-Keuls method were performed to analyze the data. RESULTS NPR-A mRNA expression was found in tissue samples of all the three areas. The mRNA expression level of NPR-A in the StV was higher than that in the NSt and vestibula (p<0.05). The difference of NPR-A mRNA expression level in the NSt and vestibula was not statistically significant (p>0.05). CONCLUSIONS Our results suggest that the StV may be the most important place for ANP to regulate endolymph fluid balance via NPR-A.