In this study, a suspension culture of recombinant Chinese hamster ovary (CHO) cells producing follicle-stimulating hormone (FSH) was used to investigate the effects of potassium ion $(K^+)$ on cell growth and FSH production. Cell growth was significantly suppressed at a $K^+$ concentration higher than 60 mM, but specific FSH productivity $(q_{FSH})$ was enhanced more than 2-fold compared to the value obtained at 4 mM $K^+$. In an attempt to alleviate the cell growth suppression at a high $K^+$ concentration, the cells were adapted at 60 mM $K^+$ in a repeated batch mode. During adaptation, the growth rate increased from 0.010 to 0.020 $h^{-1}$ and $q_{FSH}$ also gradually increased and reached 11.1 ng/($10^6$ cells h), which was even higher than that of the unadapted cells at 60 mM $K^+$. The adapted cells showed a 2.6-fold increase in maximum FSH titer at 80 mM $K^+$ compared to the unadapted cells at 4 mM $K^+$. Taken together, these results demonstrate the potential of using culture media containing cells adapted to high $K^+$ concentrations, for the enhancement of recombinant protein production.