The aim of the present study was to assess our hypothesis that the renin-angiotensin system (RAS) is responsible for cold-induced hypertension and cardiac hypertrophy. Two groups of wild-type (WT) mice and 2 groups of angiotensinogen gene knockout (Agt-KO) mice (6 per group) were used. After blood pressures (BP) of the four groups were measured 3 times at room temperature (25 degrees C), 1 WT and 1 Agt-KO group were exposed to cold (5 degrees C). The remaining groups were kept at 25 degrees C. BP of the cold-exposed WT group increased significantly in 1 week of cold exposure and rose gradually to 168+/-7 mm Hg by week 5, whereas the BP of the Agt-KO group did not increase until week 3. The cold-induced increase in BP (DeltaBP) was decreased significantly in the Agt-KO mice (19+/-3 mm Hg) compared with that of the WT mice (61+/-5 mm Hg) by 5 weeks of exposure to cold. Both WT and Agt-KO groups had cardiac hypertrophy in cold to the same extent. Agt-KO caused a significant increase in nitric oxide (NO) production. Thus, the RAS may inhibit NO formation. Chronic cold exposure decreased NO production, which may be mediated partially by activation of the RAS. These results strongly support that the RAS plays a critical role in the development of cold-induced hypertension but not cardiac hypertrophy. Moreover, the role of the RAS in cold-induced hypertension may be mediated in part by its inhibition on NO production. The findings also reveal the possible relation between the RAS and NO in cardiovascular regulation.