Aim: To compare high K+-induced contraction and optimal resting tension mea-sured by two commonly used techniques of hypertonic and isotonic K+ in aortas with and without adventitial fat from various age rats. Methods: Three age groups of rats (15, 25, and 62 weeks) were used to prepare thoracic aortic rings in which adventitial fat was either removed or left intact. High K+ (30 mmol/L)-induced contractions were observed under increasing resting tensions of 1.0, 1.5, 2.0, 2.5,3.0, and 3.5 g. Optimal resting tension was the resting tension at which the aorta showed a maximal contraction. Results: The contractions induced by 2 kinds of high K+ were significantly different. Hypertonic and isotonic K+ induced a differ-ent style of contraction, and the patte varied with different ages. At the age of 15 weeks, isotonic K+-induced contractions were greater than hypertonic K+-in-duced contractions. However, at the age of 62 weeks, isotonic K+-induced con-tractions were smaller than hypertonic K+-induced contractions. Optimal resting tensions measured by 2 kinds of high K+ were inconsistent. Optimal resting tensions in different kinds of aortic preparations from various age rats were almost a constant of 2 g, determined by isotonic K+, but a variable, determined by hyper-tonic K+. The adventitial fat could delay the development of high K+-induced contractions at different resting tensions, but had little effect on the maximal contractions. Conclusion: Hypertonic and isotonic K+ may produce different contractions resulting in differences in optimal resting tension in rat aorta.