Effects of restraint stress and its modulations by ursodeoxycholic acid(UDCA) were evaluated on some biochemical and biophysical parameters in rats. Restraint stress induced elevations in blood alkaline phosphatase (ALP). cholesterol (CHOL), aspartate transaminase (GOT), alanine transaminase (GPT), lactate dehydrogenase (LDH) levels. It was also caused adrenal hypertrophy, decrease in weight of spleen and contents of ascorbic acid in stressed rats. As a results, stress indicators such as spleen, ascorbic acid, GOT, GPT, LDH were fastly changed after imposing stress, but those such as ALP, CHOL, adrenal were induced relatively later. UDCA was tested if it has an inhibitory effect against 18-hr restraint induced stress. UDCA lowered ALP, CHOL, LDH level and also effectively elevated the ascorbic acid contents in 25 mg/kg dosage of UDCA. In organ weights. the restraint stress induced increases in spleen and adrenal were attenuated by UDCA in 50 mg/kg dosage. However. stress-induced GOT and GPT levels were unaffected by UDCA.