3HNAAG, N‐acetyl‐l‐aspartyl‐l‐glutamic acid, has been widely used as a substrate in glutamate carboxypeptidase II (GCPII) reactions, either to determine the inhibitory constants at 50% inhibition (IC50) of novel compounds or to measure GCPII activities in different tissues harvested from various disease models. The importance of 3HNAAG, combined with its current commercial unavailability, prompted the development of a reliable eight‐step synthetic procedure towards 3H2NAAG starting from commercially available pyroglutamate. Pure 3HNAAG of high molar activity (49.8 Ci/mmol) and desired stereochemistry was isolated in high radiochemical yield (67 mCi) and radiochemical purity (>99%). The identity was confirmed by mass spectrometry and co‐injection with unlabeled reference. 3HNAAG, N‐acetyl‐l‐aspartyl‐l‐glutamic acid, widely used as a substrate in glutamate carboxypeptidase II (GCPII) reactions and no longer commercially available, was prepared by a reliable eight‐step synthetic procedure starting from commercially available pyroglutamate. Pure 3HNAAG of high molar activity (49.8 Ci/mmol) and desired optical configuration was isolated in high radiochemical yield (67 mCi) and radiochemical purity (>99%).