In this study, the effects of losartan, an angiotensin II receptor antagonist, on heart rate variability and the changes of plasma corticosterone levels caused by immobilization stress were investigated. Losartan (3 mg/kg, p.o.) significantly prevented increases in plasma corticosterone levels in both losartan+acute stress and losartan+chronic stress groups. But, losartan did not prevent the diminution of the power of heart rate variability caused by stress. Our results supported the idea that the renin-angiotensin system is also involved in the stress- induced cardiovascular response, besides the autonomic nervous system. But, the effects of losartan on heart rate variability remained controversial.