Summary. Although studies on the clearance of cndogenoasly circulating amino acids had been pub]i.shed for .several mammalian and non-mamiiialian vertebrate species, few data were available for the rat. Since micropunchire studies ou the renal handling of amino acid.s are mo.st couveuiently performed on thi.s species, endogenoas clearance studies have now beeu undertakeu using automated, iou-exchange chromatographic techniques for amino acid analysis. Endogeuous plasma amino acid concentrations were remarkably similar to those in man but renal clearances of most amiuo acids were cousiderably smaller, indicating highly cfficicut rcabsorpti\'e mechanisms; the urinary excretion patterns, also, were very similar to those of uiau, although a striking exception wa.s observed in the case of taurine, wliich was cleared iu the rat at rates approachiua 408' of the glouicnilar filtration rate. It was concluded that the renal haudling of amino acids, with the exception of taurine, was very similar iu rat aud uiau. so that couclusions drawn from micropuncture experiments in rats would seem likely to hold also for man. INTRODUGTION.In recent years our knowledge of the processes involved in the reabsorption of amino acids by the kidne\-has been greatly advanced by the study of individual renal tubules using technic^ues of free-flow micropuneture (Wci.se, Eisenbach and Stolte, 1972), microinjection (Bergeron and Morel, 1969), microperfusion (Chan andHuang, 1973; Silbernagl and Dectjen, 1972;Young and Freedman, 1971) and microperfusion combined with peritubular capillary perfusion (Lingard, Rnmrich and Young, 1973a, b). As for most other micropunctux'e experiments, the rat was used in all these studies, although, unfortunately, few endogenous amino acid clearance data are available for this speciesf." Died tragically on February 20, 1974. 1 Webber (1967, however, has reported rates of amiuo acid cxcrcttou iu developing rats and approximate clearauce values cau be computed from his data. In addition. Goyer, Reynolds aud Elston (1969) have published souie preliminary' clearance values incidcutally in a study on the effect of cycloieucine aduuuistratiou ou auiiuo acid excretiou. J. LINCARD, B. TURNER, D. WILLIAMS AND J. A. YOUNGThe present study, therefore, was imdertaken to provide this missing information, since the availability of endogenons clearance data would be of great help in interpreting results obtained from micropuncture experiments on individual renal tubules. MATERIALS AND METHODS.Male albino rats weighing about .300 g were used. They were allowed free access to normal food and water up to the beginning of the experiment, and, .since plasma ainino acid concentrations may fall during starvation (Wu, 1954), all clearance studies were completed within 4 h of withdrawal of food. To minimize the risk of contamination of collected urine with hair, food particles and faeces, tbe animals were first washed, following which they were allowed to dry themselves for a few minutes. To ensure an adequate urine flow, the animals wer...