Objective To investigate the changes in plasma levels of angiotensin Ⅱ, endothelin-1 and blood pressure fluctuation during light-dark alterative period in sinoaortic denervated (SAD) rats. Methods Seventy-two male Sprague-Dawley rats underwent SAD or sham operation at the age of 12 weeks. Twenty-four-hour blood pressure and blood pressure variability (BPV) were measured in conscious, unrestrained rats 4 weeks after operation. Rats were housed in a 12 h light/12 h dark cycle (LD 12:12) for at least 10 days, then the blood samples were taken every 4 h. The plasma levels of angiotensin Ⅱ and endothelin-1 were assayed by radioimmunoassay (RIA). Results No significant difference in SBP or DBP between SAD and sham-operated rats was found. Compared with sham-operated rats, SAD rats had greater SBPV and DBPV over 24 hours (SBPV: 13.8?4.2 vs control: 6.4?1.1, DBPV:11.5?3.1 vs control: 5.3?0.8 mm Hg, all P