Nitric oxide (NO), production by microglia and astrocytes is well characterized in cells isolated from rat or mouse CNS. However, notable differences in the levels of NO generated from both hamster and human glia have led us to examine the induction of nitric oxide synthase in these species. In contrast to the high-output pathway of NO production from rat/mouse glia in response to several inflammatory and immune mediators, hamster/human glia respond with high-output NO only under specific conditions such as treatment with polyribonucleoddes or the ingestion of foreign material. For these studies, hamster or human microglia and astrocytes were either separated or used as a mixed glial culture and plated into multiwell dishes. Cells were then continually stimulated with polyribonucleotides or other mediators, such as concanavalin A, for several days and the supernatants from each well were assayed for nitrite using the Griess assay. Hamster and human microglia could be induced to produce high levels of nitrite when the cells were stimulated with agents such as poly I:poly C. The data suggest that while cytokine stimulation of hamster or human glia is generally ineffective in inducing NO, agents associated with certain membrane receptors can induce high-output NO production.