Regulation of voltage-gated K⁺ (Kv) channel expression may be involved in controlling contractility of uterine smooth muscle cells during pregnancy. Functional expression of these channels is not only controlled by the levels of pore-forming subunits, but requires their association with auxiliary subunits. Specifically, rapidly inactivating Kv current is prominent in myometrial cells and may be carried by complexes consisting of Kv4 pore-forming and KChIP auxiliary subunits. To determine molecular identity of the channel complexes and their changes during pregnancy, we examined the expression and localization of these subunits in rat uterus. RT-PCR analysis revealed that rat uterus expressed all three Kv4 pore-forming, and KChIP2 and 4 auxiliary subunits. The expression of mRNAs for these subunits was dynamically and region-selectively regulated during pregnancy. In the corpus, Kv4.2 mRNA level increased prior to parturition, whereas the expression of Kv4.1 and Kv4.3 mRNAs decreased during pregnancy. A marked increase in KChIP2 mRNA level was also seen at the late gestation. In the cervix, the expression of all three pore-forming and the two auxiliary subunit mRNAs increased at the late gestation. Immunoprecipitaton followed by immunoblot analysis indicated that Kv4.2-KChIP2 complexes were significant in uterus at the late pregnancy. Kv4.2 and KChIP2 immunoreactive proteins were present in both circular and longitudinal myometrial cells. Finally, Kv4.2 and KChIP2 mRNA levels were similarly elevated in pregnant- and non-pregnant-corpuses of one-side- conceived rats. These results suggest that diffusible factors coordinate the pregnancy-associated changes in molecular compositions of myometrial Kv4-KChIP channel complexes.