The value of gamma-aminobutyric acid (GABA) immunogold labeling in the quantification of nerve endings was studied in dissociated rat neocortex cultures. Adjacent sections were processed in 3 series according to the same staining protocol. Of all nerve ending profiles examined, 20% appeared to be GABA-positive, while 7% could not be classified. In cultures pretreated with gabaculine, GABA-positive endings were labeled more heavily. Cell bodies could always be discriminated as GABA-positive or GABA-negative. Of all neuronal cell profiles, 14% appeared to be GABA-positive. These neurons could also be identified by light microscopy using the peroxidase-anti-peroxidase method.