1. Previous results suggest that glutamine synthesis in brain could be modulated by nitrix oxide. The aim of this work was to assess this possibility. 2. As glutamine synthetase in brain is located mainly in astrocytes, we used primary cultures of astrocytes to assess the effects of increasing or decreasing nitrix oxide levels on glutamine synthesis in intact astrocytes. 3. Nitric oxide levels were decreased by adding nitroarginine, an inhibitor of nitric oxide synthase. To increase nitric oxide we used 5-nitroso-N-acetylpenicillamine, a nitric oxide generating agent. 4. It is shown that 5-nitroso-N-acetylpenicillamine decreases glutamine synthesis in intact astrocytes by= 40-50%. Nitroarginine increases glutamine synthesis slightly in intact astrocytes.