Plant high-affinity K + (HAK) transporters are divided into four major clusters. Cluster I transporters, in particular, are thought to have high-affinity for K +. Of the 27 HAK genes in rice, eight HAK transporters belong to cluster I. In this study, we investigated the temporal expression patterns during K + deficiency and K + transport activity of these eight HAK transporters. The expression of seven HAK genes except OsHAK20 was detected. Expression of OsHAK1, OsHAK5 and OsHAK21 was induced in response to K + deficiency; however, that of other genes was not. Six of the eight HAK transporters-OsHAK1, OsHAK5, OsHAK19, OsHAK20, OsHAK21, and OsHAK27-complemented the K +transporter-deficient yeast or bacterial strain. Further, the yeast cells expressing OsHAK1 were more sensitive to Na + than those expressing OsHAK5. Mutant analysis showed that the high-affinity K + uptake activity was almost undetectable in oshak1 mutants in a low-K + medium (0.02 mM). In addition, the high-affinity K + uptake activity of wild-type plants was inhibited by mild salt stress (20 mM NaCl); however, Na + permeability of OsHAK1 was not detected in Escherichia coli cells. The high-affinity K + uptake activity by leaf blades was detected in wild-type plants, while it was not detected in oshak1 mutants. Our results suggest that OsHAK1 and OsHAK5 are the two important components of cluster I corresponding to low-K + conditions, and that the transport activity of OsHAK1, unlike that of OsHAK5, is sensitive to Na +. Further, OsHAK1 is suggested to involve in foliar K + uptake.