To investigate the effects of K+ uptake on the intracellular environment, both 39K-nuclear magnetic resonance (NMR) and K+-selective electrodes were used to measure K+ activity with acute K+ loading in control and K+-adapted rats. These results were then compared with tissue K+, measured by flame photometry. There was a lower NMR K+ visibility (ratio of NMR signal to tissue content) in muscle and liver in K+-adapted rats, compared with controls before and after an acute K+ load. This lower K+ visibility in K+-adapted rats was confirmed in liver homogenate with the K+-specific electrode. In liver homogenates from control and K+-adapted rats, addition of RbCl (300 mumol/g) increased the NMR K+ signal more in K+-adapted rats (19 +/- 1.1 mumol/g) than controls (11 +/- 1.0 mumol/g, P less than 0.01). This is consistent with the displacement of K+, by Rb+, from NMR-undetected sites. These results suggest that some 10-15% of intracellular K+ may be within a compartment not detectable by NMR or electrodes and that chronic K+ loading leads to an increased capacity of this compartment.