The in- and outflow of 204Tl, 86Rb and 137Cs in a snail brain was studied. The inward rate constants determined in the presence and without ouabain decreased as follows: Tl+ greater than Rb+ greater than Cs+. The inflow rate constants differed about 2-3 fold. Rates of leakage of the cations were nearly similar, so that Tl+ approximately or equal to Rb+ greater than Cs+. A Na ouabain-sensitive outward transport in a K-free medium containing 0.1-0.2 mM Tl was as high as that in the presence of 1.6 mM external K. A further increase of external Tl inhibited the ouabain-sensitive Na transport. The intracellular distribution of Tl studied by microprobe analysis was found to be similar to that of K. There was a close resemblance of the ouabain-sensitive K-transporting system in the snail brain cells and that in other types of cells including human red cells in their selectivity to Tl+, Rb+ and Cs+. On the contrary, a passive permeability of neurons to Tl+ being similar to that of muscle cells was about 50-100 times less than a Tl+ permeability of inexcitable red cell membrane.