Introduction: Adenosine deaminase (ADA; E. C. (Enzyme Commission) 3.5.4.4), an enzyme which catalyses the deamination of adenosine to inosine in purine reutilization, is widely distributed in human and animal tissue. The ADA is abundantly present in thymus, spleen, appendix and Peyer's patches and is considered required for the differentiation and maturation of lymphoid cells, especially T cells. ADA activity is considered an index of the immune function. Interrelations between stress and ADA activity have not been studied yet. The present study was conducted to determine whether and how restraint stress influences ADA activity in rats.Methods: Male Wister rats, 12 weeks old, were used. Restraint stress was caused in individual rats by wrapping wire gauze around them in the prone position at 4 hours/day (9 AM to 1 PM) for 3 consecutive days. Thirty minutes following restraint stress on the third day, blood samples were obtained from the cardiac ventricle by syringe, centrifuged for 10 minutes at 2,000 g and serum was separated and frozen at -80°C. The kit for ADA activity, AD Test MARUHO (Maruho Co. Ltd. Osaka, Japan), consisting of buffered substrate, phenol color reagent, alkali-hypochlorite reagent, and standard fluid was used for this purpose. This is a micro-method with catalyzed indophenol reaction for determining ammonia separated by ADA. The mean of duplicate determinations was taken as ADA activity.Results: Serum ADA activity was higher in the restrained than control rats.Discussion: Increase in ADA activity of restrained rats would thus appear closely related to enhance maturation and differentiation of suppressor T lymphocytes.ADA activity can be measured more easily using an auto-analyzer and divide from two isoenzyme, ADA1 and ADA2. Although additional study is needed to clarify the effects of stress on the activity of each of these isoenzyme and significance disease diagnosis, the present results should prove significant in psychoneuroimmunology.