Renal tubular reabsorption of taurine, gamma-aminobutyric acid (GABA), and beta-alanine was studied in vivo et situ by continuous microperfusion of single proximal tubules of the rat. In each case, reabsorption was much slower than that for other amino acids that have been studied. With a concentration of 0.1 mmol/l in initial load was reabsorbed over perfusion distance of 3 mm. Taurine reabsorption saturated with only 2.17 mmol/l in initial perfusate. Assuming simple two-parameter kinetics, upper limits for Km of 0.54 mmol/l and forVmax of 0.59 pmol-cm-1--s-1 for tubular reabsorption of taurine were estimated. High (20 mmol/l) concentrations of taurine or beta-alanine in perfusate completely inhibited GABA reabsorption, but L-phenylalanine (20 mmol/l) had no significant effect. The results indicate that the three amino acids are reabsorbed slowly from the proximal tubule by what may be a common transport system. This system appears to have a high affinity but low capacity and to be different from other known renal tubular transport systems for amino acids.