The rate of transport of phenylalanine and leucine, pertinent amino acids of System L, has been measured in SV40 3T3 cells as a function of the presence of Na+ ions during the reloading phase that precedes the influx determination. The presence of Na+ ions during the reloading phase resulted in an increase of the subsequent substrate influx through System L. This effect was related to the intracellular Na+ level and was found to be independent by the presence of a chemical sodium gradient outside-inside during influx determination; furthermore, this effect could not be ascribed to a difference between control and Na+-treated cells in the internal levels of those amino acids that participate in the exchange phenomena of transport System L. The transport of phenylalanine appeared to have the ability to accept Li+ for Na+ substitution in the 'trans' position. The presence of Na+ ions in the 'trans' position was not required to optimize the transport of System A-reactive substrates, whose influxes are dependent on the presence of the cation in 'cis' position. Analysis of the relationship between influx and substrate concentration indicated that the Na+-dependent increase of substrate influx was associated with an enlarged capacity of the high-affinity component of transport System L.