Endothelin-1 (ET-1) is a peptide that mediates blood pressure. ECE-1b-C-338A is a polymorphism in the ECE-1 gene that alters ET-1 synthesis. Thus, the goal of this study was to investigate the influence of the ECE-1b-C-338A polymorphism on plasma and urinary ET-1 levels, as well as on aerobic exercise training (AEX)-induced changes in these variables. Outcome variables were assessed before and after 24 weeks of AEX in hypertensive adults. Separate ANCOVAs were used to compare differences in outcome variables. Only AA homozygote men had higher plasma ET-1 levels. After AEX, there were no significant changes in the main outcome variables. Neither were these changes significantly different between genotype groups. Results suggest that this polymorphism is associated with plasma ET-1 levels in men but not with urinary ET-1 levels. Additionally, this polymorphism does not affect AEX-induced changes in plasma and urinary ET-1 levels.