The kinetics of Rb+ transport in Saccharomyces cerevisiae depended on the K+ content of the cells and on K+ starvation, as follows. In cells with normal K+ (grown at millimolar K+), Rb+ transport was regulated by internal K+. The loss of K+ first decreased the Km and later increased the Vmax of Rb+ transport. K+ starvation of normal-K+ cells for 4-5 h decreased the Km of Rb+ transport below the minimum observed after K+ loss. During this time Eadie-Hofstee plots of Rb+ transport suggest that the existing system was converted into a new one with a higher affinity. Growth at 10 microM K+ only required the system triggered by K+ loss, and the system expressed in K+-starved cells was not expressed under these conditions.