Melatonin but not serotonin or tryptophan inhibited iNOS promoter activity. RAW 264.7 cells transfected with luciferase expression vector containing iNOS promoter were pretreated with melatonin (A), serotonin (B), or tryptophan (C) (1 mM each) for 30 minutes followed by LPS, or LPS/IFNγ for 8 hours. iNOS promoter activity was measured as luciferase activity expressed as relative light unit (RLU)/μg proteins. Each bar denotes mean ± SEM of 3 experiments.