AIM: To study the immunoregulatory effect of 1,25-dihydroxyvitamin-D3 Von dominant Th1 response in rats.METHODS: Sixty adult Lewis rats were randomized into three groups.Rats in group 1 (n=25) were treated with 1,25-(OH)2D3 first and then challenged with LPS,rats in group 2 (n=2S) were treated with vehicle first and then challenged with LPS.Ten animals in groups 1 and 2 were preserved for mortality observation.The remaining animals were injected (i.p) with endotoxin,24 h after the last administration of 1,25-(OH)2D3 and vehicle.Rats in group 3 (n=10) were treated with 1,25-(OH)2D3 only.Serum IL-12,IFN-y,IL-2 and IL-4 levels were measured and target gene of 1,25-(OH)2D3 on Th cells was studied after 6 h.Gene abundance was verified by real-time quantitative PCR.RESULTS: No death occurred in rats pretreated with 1,25-(OH)2D3 after LPS injection.Death occurred 9 h after LPS injection in rats pretreated with the vehicle,and the number of deaths was 5 within 24 h,with a mortality rate of 50%.There was no change in the number of deaths within 96 h.Six hours after endotoxin stimulation,serum IL-12 and IFN-y levels decreased significantly in rats pretreated with 1,25-(OH)2D3 as compared with those in rats pretreatecl with the vehicle.The serum content of these two cytokines was very low in rats not challenged by endotoxin,and there was a significant difference as compared with the previous two groups.CONCLUSION: 1,25-(OH)2D3 attenuates injury induced by the lethal dose of LPS,regulates Th1 and Th2 cells at the transcription level,and dominantly responds to cytokine production in rats.