The regulation of the transport of neutral amino acids across the cell membrane by adaptive mechanisms has been studied in cultured human fibroblasts. Among the three transport systems (A, ASC, and L) individually discriminated, only the Na+-dependent System A was subject to adaptive regulation, showing enhancement of its activity when the cells were incubated under conditions of amino acid shortage (derepression phase) and decrease of its activity when the cells were exposed to a medium supplied with Site A-reactive amino acids (repression phase). Starvation-induced derepression of transport activity and its reversal by amino acid refeeding required active RNA and protein synthesis. Derepression involved an early mRNA synthesis which started within 30 min from the abrupt change in extracellular amino acid concentration and apparently lasted 90 min. The transcribed mRNA was rather stable and translatable for a few hours (presumably into transport proteins) in cells maintained in the absence of amino acids. Repression by amino acid refeeding also involved an early mRNA synthesis, the product of its translation being presumably a protein capable of causing degradation or inactivation of transport proteins. The rate of decay in transport activity of previously derepressed cells was somewhat faster in the presence of added Site A-reactive amino acids than in their absence. A model is proposed in which the concentration of Site A-reactive amino acids affects transport activity of System A by modulating transcription of mRNA species coding for transport proteins and their putative inactivators and by regulating the efficiency of transport protein inactivation at the cell membrane.