Aim To investigate the changes of Ca²⁺-activated K~+(KCa) channels of rats aortic smooth muscle cells with aging.Methods The aortic smooth muscle cells from rats of three groups(1,6,20 months,n=20 respectively) was digested by enzyme and studied the activity of KCa in the rats aortic smooth muscle cells by the Patch-clamp technique.The signal channel open probability(Po),open dwell-time(To),close dwell-time(Tc) and urrent ampitude(AM) were recorded.Results(1)The conductance of the rats aortic smooth muscle cells of 1,6,20 months was 192±47 ps,177±56 ps,163±35 ps respectively.There was no significant difference in each group(p>0.05).But the AM of the rats aortic smooth muscle cells decresed with aging.(2)The Po of KCa in the rats aortic smooth muscle cells was 0.009±0.001,0.015±0.004 respectively,The Tc was 2260±653,2512±185 respectively in 20 month-old rats and in 1 month-old rats under inside-out patch.The activity of KCa had significant difference between two groups(p<0.01).(3)The sensitity of KCa channel to Ca²⁺ in the aortic smooth muscle cells from different month-old rats: After adding Ca²⁺(5×10~(-7),10~(-6),10~(-5) mol/L) to cytoplasm,the Po incresed and the Tc decresed in each group.While comparison of different month-old rats,the Po and the To of 20 month-old rats was less than that of 6 month-old rats and 1 month-old rats.Conclusion The activity of KCa reduced in rats aortic smooth muscle with aging