Abstract: Objective To investigate the effect of ascorbic acid (AA) and vitamin D₃ (VD₃) on the level serum of 1,25-(OH)₂D₃ and ulcerative colitis in guinea pigs.Methods Thirty-two guinea pigs were randomly divided into 4 groups (8 in each group):a control group (AA of 120 mg/kg and VD₃ of 2.8 μg/kg),a model group (AA of 120 mg/kg and VD₃ of 2.8 μg/kg),and high and low AA groups (AA of 240 mg/kg and VD₃ of 2.8 μg/kg,AA of 8 mg/kg and VD₃ of 2.8μg/kg).All the guinea pigs were fed with special feed containing no AA and vitamin D and treated with various dosages of AA and VD₃ by modified gastric lavage for 7 weeks;then the guinea pigs of control group were supplied with distilled water and those of other groups with 3% dextran sodium sulfate solution as drinking water for 3 days.By the end of the treatments,all the guinea pigs were sacrificed for detections of relevant indicators.Results Compared with those in the control group,significantly increased scores of colon's disease activity index (DAI),gross morphology and histopathology,decreased serum contents of 25-(OH)D₃ and 1,25-(OH)₂D₃,decreased expressions of cytochrome P450 2R1 (CYP2R1) in liver tissues and cytochrome P450 27B1(CYP27B1) in renal tissues,and increased serum interleukin-6 (IL-6) (80.21±6.22 ng/ml) and tumor necrosis factor alpha (TNF-α) (174.08±11.04 ng/ml) were detected in the model group (P<0.05 for all).Compared with those in the model group,significantly increased scores of colon DAI and gross morphology,decreased serum 25-(OH)D₃,and increased serum IL-6 (93.23±4.76 ng/ml) and TNF-α (208.46±14.15 ng/ml) were observed in the low AA group (P<0.05 for all).Compared with those in the low AA group,significantly lower scores of colon DAI and gross morphology,higher serum 25-(OH)D₃ and 1,25-(OH)₂D₃,higher expressions of CYP2R1 in liver tissues and CYP27B1 in renal tissues,and lower serum IL-6 (75.20±3.85 ng/ml) and TNF-α (167.83±11.85 ng/ml) were detected in the high AA group (P<0.05 for all).Conclusion Ascorbic acid can effectively promote VD₃ to be hydroxylated into an active form of 1,25-(OH)₂D₃ in the appropriate range and AA combined with VD₃ has an intervention effect on ulcerative colitis induced by dextran sulfate sodium in guinea pigs.