Suspension cells of sugar cane were used as a model system for cells of higher plants to study the mechanism of L-arginine uptake. The uptake system is specific for the L-arginine molecule in the fully ionized state, i.e. delta-guanidino group and alpha-amino group positively charged and carboxyl group negatively charged. This was concluded because the Km value for uptake increased strongly for: (a) L-arginine analogues which lack the charged carboxyl group (L-arginine methyl ester, agmatin); (b) L-arginine analogues, which lack the charged alpha-amino group (L-arginine acid, gamma-guanidinobutyric acid); (c) L-arginine analogues, which lack the charged delta-guanidino group or gamma-guanidinoxy group (L-citrulline, L-canavanine at neutral and alkaline pH-values). The importance of the positive charge of the delta-guanidino group or gamma-guanidinoxy group was further documented by Km values for L-arginine and L-canavanine at different pH values. Only at pH values where the gamma-guanidinoxy group is protonated, was there an effective uptake of L-canavanine and effective competition of L-canavanine with L-arginine. The length of the L-arginine molecule was less important: slightly larger (L-homoarginine) or shorter analogues (L-lysine) were taken up rather well. A spatial rearrangement at the alpha-carbon (D-ariginine) was, however, not tolerated. The uptake of L-arginine proceeds by electrogenic uniport, there is no evidence for symport or antiport of another molecule (though L-canavanine uptake at neutral pH value causes a transient alkalinization of the suspension medium). Charge equilibration is brought about by efflux of protons and potassium ions.