Nitric oxide (NO) is considered to play a crucial role in the development of various pathological processes in the CNS, such as neuronal degeneration, inflammation and demyelination. In order to search for the agents which suppress NO production in the CNS, we examined the effects of one of the agents which elevate cyclic AMP production, phosphodiesterase inhibitors (PDEIs), on NO production by glial cells in vitro. All the types of PDEIs, from type I- to V-specific and non-specific, suppressed the production of NO by mouse microglia and astrocytes stimulated with lipopolysaccharide, in a dose-dependent manner. Suppression of inducible NO synthase by PDEIs was confirmed by the expression of mRNA by RT-PCR. Although it required 10 μM or higher concentration to effectively suppress NO production in vitro, certain combinations of three different PDEIs synergistically suppressed NO production by astrocytes at 1 μM which could be obtained in vivo at usual therapeutic doses. Similary, combinations of three PDEIs at 1 μM synergistically increased intracellular cAMP in astrocytes. The suppressive effects of PDEIs on NO production were abolished by addition of tumor necrosis factor α (TNFα). Thus, the main suppression mechanism of NO might be indirect through suppression of TNFα. Since some PDEIs are reported to pass through the blood-brain-barrier, the combination of three PDEIs may be worth trying in neurological diseases, such as multiple sclerosis, human immunodeficiency virus-related neurological diseases and other neurodegenerative disorders in which NO may play a crucial role.