【Abstract】ObjectiveTo investigate the effects of endogenous photodynamic therapy (PDT) on intracellular cAMP and cGMP concentrations of human colon carcinoma cell lines SW480. MethodsSW480 cells were divided into control group, light group, δaminolevulinic acid （ALA） group (ALA group) and endogenous PDT group (ALAPDT group). Intracellular cAMP and cGMP concentrations of each group were detected by radioimmunoassay at 30, 60, 90 and 120 min after irradiation. ResultsThere was a significant increase in intracellular cAMP concentration of ALAPDT group at 30 min after irradiation (P＜0.001) and sequent decrease, but intracellular cAMP concentrations of ALAPDT group at 60, 90 and 120 min after irradiation had no statistical difference than the other groups (Pgt;0.05). Intracellular cGMP concentration of different time point of each group was not significantly different. ConclusionThese results indicate that the cytoprotection of SW480 cell are produced by an instantaneous increase in the intracellular cAMP concentration while endogenous PDT is killing SW480 cell.