We evaluated the association between insulin resistance and the angiotensin-converting enzyme (ACE) insertion (I)/deletion (D) gene polymorphism in a group of older hypertensive subjects (63 ± 1 yr, n = 35) before and after a 6-mo aerobic exercise program (AEX). Insulin sensitivity index (S_I), assessed by the frequently sampled intravenous glucose tolerance test, was significantly ( P = 0.0001) increased following AEX. In addition, there was a significant ( P = 0.001) interaction between AEX and ACE genotype. S_I increased significantly ( P < 0.05) more in those with the II (2.5 ± 0.8 μU × 10⁻⁴ · min⁻¹ · ml⁻¹) ACE genotype compared with both the DD and ID (0.7 ± 0.1 and 0.7 ± 0.2 μU × 10⁻⁴ · min⁻¹ · ml⁻¹, respectively) ACE genotypes. Similarly, there was a significant ( P = 0.036) decrease in the acute insulin response to glucose (AIR_G) and a significant ( P = 0.05) interaction between AEX and ACE genotype. AIR_G decreased significantly ( P < 0.05) more in those with the II (−17.6 ± 5.6 mU/ml) ACE genotype compared with both the DD and ID (−1.4 ± 6.2 and −3.6 ± 2.5 mU/ml) ACE genotypes. In conclusion, we demonstrated that those older hypertensives with the ACE II genotype have the greatest improvement in insulin action following AEX.