Phosphate activated glutaminase comprises two kinetically distinguishable enzyme forms in cultures of cerebellar granule cells, of cortical neurons and of astrocytes. Specific activity of glutaminase is higher in cultured neurons compared with astrocytes. Glutaminase is activated by phosphate in all cell types investigated, however, glutaminase in astrocytes requires a much higher concentration of phosphate for half maximal activation. One of the products, glutamate, inhibits the enzyme strongly, whereas the other product ammonia has only a slight inhibitory action on the enzyme.