Amino acid transport was characterized in stage 6 Xenopus laevis oocytes. Most amino acids were taken up by the oocytes by way of both Na+-dependent and saturable Na+-independent processes. Na+-dependent transport of 2aminoisobutyric acid (AIB) was insensitive to cisor trans-inhibition by the System A-defining substrate 2-(methylamino)-isobutyric acid (MeAIB), although threonine, leucine, and histidine were found to be effective inhibitors, eliminating greater than 80% of Na* -dependent AIB uptake. Lack of inhibition by arginine eliminates possible mediation by System B",+ and suggests uptake by System ASC. The Na +-dependent transport of characteristic System ASC substrates such as alanine, serine, cysteine, and threonine was also insensitive to excess MeAIB. Evidence to support the presence of System B",+ was obtained through inhibition analysis of Na+-dependent arginine transport as well arginine inhibition of N a tdependent threonine uptake. The Na+-independent transport of leucine was subject to trans-stimulation and was inhibited by the presence of excess phenylalanine, histidine, and, to a lesser extent, 2-amino-(2,2,1)-bicycloheptane-2-carboxylic acid (BCH). These observations are consistent with mediation by System L. The characteristics of Na+-independent uptake of threonine are not consistent with assignment to System L, and appear to be reflective of Systems asc and b",+. In its charged state, histidine appears to be transported by a carrier similar in its specificity to System y+, but is taken up by System L when present as a zwitterion.