The presence of homocysteate and homocysteine sulfinate was demonstrated in extracts prepared from cultures of rat cortical and cerebellar astrocytes as well as from C6 glioblastoma cells by o-phthalaldehyde derivatization and subsequent HPLC analysis. Homocysteate-like immunoreactivity was found in cultured cortical astrocytes by postembedding immunocytochemistry at the level of light microscopy. These findings support the notion of a glial localization of the excitatory transmitter candidate homocysteate.