Abstract The cerebral influx rates of fifteen amino acids were measured directly in living rats by means of a new technique which makes it possible to maintain a constant specific activity of a radioactively labelled amino acid in the bloodstream. A wide variation in the influx rates of the amino acids was found. These rates differed from those found by other workers using in vitropreparations, but are consistent with the theory that amino acids enter the brain mainly by carrier mediated transport processes with a high degree of specificity. There are a number of important differences between the behaviour of the transport processes in vivo and in vitro. The influx rates of the various amino acids were directly proportional to their concentra­tions in blood plasma (over the range of concentrations studied). All the nutritionally essential amino acids had relatively high influx rates as did other amino acids which the brain does not seem to be able to synthesize. On the other hand, amino acids that the brain can readily synthesize and two amino acids which are not normally found in mammalian tissues had low influx rates.