OBJECTIVE To study the changes of nitric oxide(NO) and nitric-oxide synthase(NOS) in the development of cold-induced hypertension (CIH). METHODS Sixty male Sprague-Dawley adult rats were used. Thirty were exposed to cold (4 +/- 1) degrees C as cold-treated group while the other 30 were at (25 +/- 1) degrees C as controls, 4 hours a day for 6 weeks for both groups. Systolic blood pressure (SBP) and heart rate were measured twice every week. Each group was further subdivided into three groups, 10 rats each. A subgroup of the cold-treated and control rats were sacrificed at 2, 4 and 6 week. Plasma was saved to measure superoxide dismutase (SOD) and malondiadehycle (MDA), while heart was homogenated to measure NO, NOS. RESULTS 1 SBP increased during 6 weeks of exposure to cold. From the second week, SBP of cold-treated group [(94.16 +/- 3.81) mm Hg] was significantly greater than that of control group [(88.77 +/- 4.45 mm Hg), P<0.01]. The highest SBP level was achieved at the sixth week [(116.78 +/- 3.79)mm Hg, P<0.01]. 2 Compared to the control group, SOD in cold-treated group decreased significantly from the second week, and maintained throughout the time of exposure to cold (P<0.05). MDA levels did not differ significantly between cold-treated and control groups though it increased mildly during 6 weeks of cold exposure (P>0.05). Heart NOS in cold-treated group decreased significantly from the fourth week to the sixth week. And a mild decrease was observed in heart NO of cold-treated group during 6 weeks of exposure (P>0.05). CONCLUSIONS Cold-induced hypertension is induced in rats after repeated exposure to cold. The levels of NOS, NO decrease accordingly to the rise of blood pressure. This indicates that the dysfunction of NO and NOS is involved in the development of CIH.