1. In the snail suboesophageal ganglion 5-hydroxytryptamine (5-HT) is taken up by an active transport process that requires extracellular sodium and is inhibited by high concentrations of ouabain (1–10 mM). 2. The action of oubain in this system is complex, since even at 0°C, where the sodium pump is inoperable, significant inhibition of 5-HT could be demonstrated. 3. The Na +, K +-ATPase was maximally inhibited by 0.1 mM ouabain, a concentration that does not affect 5-HT uptake, thereby probably excluding a direct linkage between this enzyme and the 5-HT uptake process. 4. Moreover, the presence of 0.1 mM ouabain was sufficient to increase the intracellular sodium concentration of the ganglion by 91% without affecting 5-HT uptake, suggesting that the directed sodium gradient present in cells is not the likely energy source nor is it related to the mechanism of 5-HT uptake by snail ganglia.