The development of gamma-glutamyl transpeptidase (GGT) activity in neurones and glial cells was studied in primary cell cultures derived from the cerebral hemispheres of chick embryos. GGT activity was found in both basic types of nervous tissue cells. It was always higher in glial cell cultures, where it was up to 2.3-fold the values in neurone-enriched cultures. If the culture medium contained foetal calf serum, the GGT activity of both types of nerve cells was higher than in the presence of inactivated calf serum. Comparison with the in vivo situation showed that the level of GGT activity in nerve cell cultures was significantly lower. Between the seventh day of embryogenesis and the third day of postnatal development of the nerve cells, there were marked differences between the GGT activity of cells maintained under in vitro conditions and cells of the same age in brain tissue homogenate. GGT activity in cerebral hemisphere homogenates from a 17-day-old embryos amounted to 4-fold the activity in a primary glial cell culture and to 16-fold the value in a neurone-enriched culture from hemispheres at the same stage of development.