1. 1. Slices of rat cerebral cortex incubated aerobically at 37°C in Krebs-Ringer-bicarbonate solution accumulated 3,4- l-dihydroxyphenylalanine ( l-DOPA) against its concentration gradient. With 1 mM l-DOPA in the medium, tissue-water/medium concentration ratios of about 6:1 are reached, which are modified by the presence of other amino acids in the medium. 2. 2. Kinetic analysis suggested that l-DOPA influx into brain cells occurred by at least two saturable processes, which show apparent K m values in the range of 10 −3 M and 10 −5 M, respectively. 3. 3. Prior incubation of the slices in Na +-free (choline-containing medium at 37°C depressed their subsequent uptake of l-DOPA in normal Na +-containing medium; this inhibition did not appear when the preincubation was carried out at 0–4°C. Besides this effect of preincubation, most of l-DOPA influx into brain slices was independent of the actual concentration of Na + in the medium: the two saturable processes described in this article behaved similarly in this respect. 4. 4. Most of l-DOPA uptake by the high- K m process is mediated by an agency that resembles the Na +-independent L system described in Ehrlich cells (Oxender, D.L. and Christensen, H.N. (1963) J. Biol. Chem. 238, 2686–2699), both in its specificity and in its participation in exchange phenomena. A lesser component of uptake by a type A mediation is also suggested as contributing to the high- K m process. 5. 5. The kinetic and specificity properties of the low- K m process of l-DOPA uptake suggest a similarity between its mediation and that of the high-affinity systems for l-tyrosine and l-tryptophan found in brain tissue preparations (Belin, M.F. and Pujol, J. F. (1973) Experientia 29, 411–413; Bauman, A., Bourgoin, S., Benda, P., Glowinski, J. and Hamon, M. (1974) Brain Res. 66, 253–263).