GAD mRNA was detected in human and monkey brain postmortem by in situ hybridization with an [35S]-labelled copy RNA corresponding to a 2.7-kb fragment of the coding region of human GAD mRNA. A characteristic and reproducible pattern of hybridization was obtained with the anti-sense, but not the sense probe in both monkey and human brain. Microscopic examination of tissue sections showed that only neuronal perikarya, not glial cells, were labelled. The data confirm the heterogeneity of GAD mRNA distribution reported in rodent brain and non-human primate brain.