3-[ 123 I]Iodo-L-a-methyl tyrosine ( 123 I-IMT) is used for diagnosis and monitoring of brain tumours by means of singlephoton emission tomography. As recently shown, 123 I-IMT is predominantly mediated into rat C6 glioma cells by sodiumindependent system L for large neutral amino acids. Until now, 123 I-IMT transport in non-neoplastic glial cells has not been examined. Therefore, the aim of this study was to examine the cellular pathways and precise transport kinetics of 123 I-IMT uptake into astrocytes of neonatal rats. In particular sodiumindependent 123 I-IMT transport into neonatal astrocytes was compared with sodium-independent 123 I-IMT uptake into neoplastic rat C6 glioma cells.Competitive inhibition experiments showed that 123 I-IMT is exclusively transported via sodium-independent system L into the neonatal astrocytes (92%). Kinetic analysis of sodium-independent 123 I-IMT uptake into neonatal astrocytes and into C6 glioma cells revealed apparent Michaelis constants K M 13.9^0.5 mM and K M 33.9^4.1 mM, respectively, which are in the same range of K M values as those recently determined for amino acid transport into neoplastic and non-neoplastic glial cells. Indeed, the K M values in the micromolar range correspond to the expression of the LAT-1 subunit of system L both in the neonatal astrocytes and in C6 glioma cells. However, sodium-independent maximum transport velocities (V max ) differed significantly between neonatal astrocytes and C6 glioma cells (11.1^0.3 and 39.9^3.3 nmol/mg protein/10 min, respectively).