The experiments were carried out to test whether acid-sensing ion channel 1a and 3 (ASIC1a and ASIC3) were expressed on the primarily cultured type I cells of rat carotid bodies (CBs) and whether the expression of the channels was affected by acid stimulation. The Sprague-Dawley rats of either sex (50-100 g) were used. The CBs were isolated and primarily cultured. The immunofluorescent technique was used to detect the expression of tyrosine hydroxylase (TH), a specific marker of type I cells, in order to identify the type of the cultured cells. The double-label immunofluorescent technique was used to detect the expression of ASIC1a and ASIC3 on the TH-positive type I cells. To detect the influence of acid stimulation on the expressions of ASIC1a and ASIC3, each batch of primarily cultured cells were randomly divided into pH7.3 group (control group), pH6.8 group and pH6.2 group (n=9 in each group). The cells in above three groups were treated with pH7.3, pH6.8 and pH6.2 mediums for 24 h, respectively, and then the mRNA expressions of ASIC1a and ASIC3 in type I cells were detected by semi-quantitative RT-PCR technique. The results showed that more than 93% of the primarily cultured CB cells were TH-positive, indicating that most of the cultured cells were type I cells. Furthermore, all TH-positive cells expressed ASIC1a or ASIC3. After the cells were treated with acid stimulation, the amount of ASIC1a mRNA did not change significantly (P>0.05 vs control group); the amount of ASIC3 mRNA had no significant change in pH6.8 group compared with that in control group, but decreased significantly in pH6.2 group (P<0.01 vs control group, P<0.05 vs pH6.8 group). It is concluded that acid stimulation down-regulates the level of ASIC3 mRNA, but has no effect on the level of ASIC1a mRNA.