The transport of alkali metal cations across vesicle bilayer membranes mediated by a series of 21 synthetic transporters was investigated by a pH-stat technique. The relative activities of the most active synthetic transporters were comparable to valinomycin but a factor 2-20-fold less active than gramicidin. Transport activity is controlled by structural variables; the most active materials have hydrophilic head groups, a balance of hydrophilic and lipophilic groups in the wall units, and overall length compatible with the bilayer thickness. Cation selectivity among the alkali metals, inhibition of transport by a competing cation, apparent kinetic order in transporter, and the ability of transporters to move between vesicles were determined. Significant inhibition of cation transport by competing cations, non-Eisenman selectivity patterns, and zero-order transport kinetics are proposed as criteria to recognize channel-like transporters. By these criteria, six transporters apparently act as channels, and a further four apparently act as carriers of cations. Structural variables control the mode of action of the synthetic transporters, with channel-like behavior most closely associated with columnar structures.