To test whether changes in sympathetic nervous system (SNS) activity or insulin sensitivity contribute to the heterogeneous blood pressure response to aerobic exercise training, we used compartmental analysis of [³H]norepinephrine kinetics to determine the extravascular norepinephrine release rate (NE₂) as an index of systemic SNS activity and determined the insulin sensitivity index (S_I) by an intravenous glucose tolerance test, before and after 6 mo of aerobic exercise training, in 30 (63 ± 7 yr) hypertensive subjects. Maximal O₂ consumption increased from 18.4 ± 0.7 to 20.8 ± 0.7 ml · kg¹ · min¹ (P = 0.02). The average mean arterial blood pressure (MABP) did not change (114 ± 2 vs. 114 ± 2 mmHg); however, there was a wide range of responses (19 to +17 mmHg). The average NE₂ did not change significantly (2.11 ± 0.15 vs. 1.99 ± 0.13 µg · min¹ · m²), but there was a significant positive linear relationship between the change in NE₂ and the change in MABP (r = 0.38, P = 0.04). S_I increased from 2.81 ± 0.37 to 3.71 ± 0.42 µU × 10⁴ · min¹ · ml¹ (P = 0.004). The relationship between the change in S_I and the change in MABP was not statistically significant (r = 0.03, P = 0.89). When the changes in maximal O₂ consumption, percent body fat, NE₂, and S_I were considered as predictors of the change in MABP, only NE₂ was a significant independent predictor. Thus suppression of SNS activity may play a role in the reduction in MABP and account for a portion of the heterogeneity of the MABP response to aerobic exercise training in older hypertensive subjects.