The L-glutamate decarboxylase (GAD) levels of 70 rat brain nuclei were determined. The distribution of GAD is uneven. High AAD activity was found in the substantia nigra, the colliculi, some hypothalamic nuclei (e.g. dorsomedial) some nuclei of the limbic system (e.g. tractus diagonalis and accumbens) and the medial forebrain bundle. The reticular part of the substantia nigra had the highest activity, two-fold higher than any other nucleus. Low GAD activity was found in the nuclei of the pons, the hypothalamic median eminence and the reticular formation. Moderate activity was measured in most areas studied. The biochemical mapping of GAD is discussed in relation to possible GABA pathways in the brain.