Male C57 mice received 10 consecutive daily intraperitoneal injections of melatonin, 5-methoxytryptamine or 5-methoxytryptophol (5mg/kg body weight). Control mice received the alcoholic saline vehicle. All mice were sacrificed 24 hours after the last injection. Following extraction of RNA from peritoneal exudate cells (PEC) and splenocytes, the level of gene expression was analyzed with the reverse transcription-polymerase chain reaction (RT-PCR). The results revealed that melatonin up-regulated the level of gene expression of transforming growth factor-beta (TGF-beta), macrophage-colony stimulating factor (M-CSF), tumor necrosis factor-alpha (TNF-alpha) and stem cell factor (SCF) in PEC, and the level of gene expression of interleukin-1beta (IL-1beta), M-CSF, TNF-alpha, interferon-gamma (IFN-gamma) and SCF in splenocytes. 5-Methoxytryptamine augmented the level of gene expression of TGF-beta, M-CSF and SCF in PEC, and the level of gene expression of IL-1beta, TNF-alpha, IFN-gamma, M-CSF and SCF in splenocytes. 5-Methoxytryptophol elevated the level of gene expression of TNF-alpha, IL-1beta, TGF-beta and M-CSF in PEC, and the level of gene expression of inducible nitric oxide synthase (iNOS), IL-1beta, M-CSF, TNF-alpha, IFN-gamma and SCF in splenocytes.