1. The influence of sodium and potassium on the accumulation of 5-hydroxytryptamine (5-HT) by rat blood platelets was investigated.2. An absolute dependence of 5-HT uptake on the sodium concentration in the medium was found.3. Removal of potassium reduced the uptake by about 60%. High concentrations of potassium inhibited sodium-dependent accumulation.4. The observations have been discussed in terms of a carrier-mediated transport process for 5-HT operating in the platelet membrane.