Objective: To observe the effects of ursodeoxycholic acid (UDCA) on gallbladder stone formation, hepatic aminopeptidase N (APN) mRNA level and APN activity and total protein content in bile of New Zealand rabbits, and to investigate the mechanism of UDCA in preventing gallbladder stone formation. Methods: (1) Animal model: 30 New Zealand rabbits. In the control group, 10 rabbits were fed a normal diet; in the stone group, 10 rabbits were fed a lithogenic diet containing 1.2 g?d-1 cholesterol for 4 weeks; in the UDCA group, 10 rabbits were fed a UDCA diet containing both 1.2 g?d-1 cholesterol and 0.045 g?d-1 for 4 weeks. (2) APN expression: Primers were designed according to the cDNA sequence of rabbit APN gene, total hepatic RNA was extracted, and the changes of hepatic APN mRNA level were detected by RT-PCR. (3) APN activity: Assayed by chemical method. (4) Lipid and total protein content in bile: detected by biochemical method. (5)Cholesterol saturation index (CSI) calculation:calculated by Carey table. Results: 6 (6/10) gallbladder stones and 9 (9/10) cholesterol crystals appeared in the stone group, no stones and crystals appeared in the UDCA and control groups; CSI (0.80±0.09 in the control group, 1.38±0.27 in the stone group and 1.18±0.11 in the UDCA group) and total protein concentration (1.25±0.49 in the control group and 4.5±0.49 in the stone group) in the gallbladder bile of the control and UDCA groups. 0.49, 4.58±0.93 in the stone group and 3.12±0.78 in the UDCA group) were significantly lower than those in the stone group (all P less than 0.05), while the APNmRNAIOD ratio in the stone group (0.65±0.18 in the control group, 1.08±0.35 in the stone group and 0.72±0.23 in the UDCA group), APN activity [control group (6.10±2.53) U?L-1, stone group (23.64±10.36) U?L-1 and UDCA group (8.01±2.89) U?L-1] were significantly higher than those of the control and UDCA groups (all P less than 0.05). Conclusion:The above results suggest that UDCA prevents the formation of cholesterol stones mainly by reducing the expression of hepatic APN and enzyme activity and inhibiting the lithogenic effect of APN.