Comparisons were made of the inhibitory effects of ouabain and potassium-deficient media on the transport of sero tonin and norepinephrine by synaptosomes. The following parallel effects were observed in both instances. Substrate introduced into the media was transported for up to 5 min, then stopped. Pre liminary incubation of synaptosomes in the above media, before substrate was introduced into the beaker, prevented the initial transport. Inhibition of transport was dependent upon the con centration of external sodium, being greater with 143 mM than with 50 mM sodium. The inhibitory effect of ouabain on transport was antagonized by high concentrations of potassium. It is concluded that ouabain blocks transport by blocking a potassium-requiring factor in transport, possibly (Na+ + K+)-ATPase. Thus, potassium deficiency and ouabain inhibit transport indirectly by inhibiting the energy-producing aspect of transport. The action of ouabain may include a direct component, especially in the case of norepinephrine.