Extracellular superoxide dismutase (EC-SOD) is a major superoxide scavenger and may be important to normal vascular function and cardiovascular health. We analyzed family data from 610 healthy Australians to detect and quantify the effects of genes on normal variation in plasma levels of EC-SOD and to test for pleiotropy with plasma nitric oxide (NO) and apolipoprotein A-I (apoA-I). Using maximum-likelihood-based variance decomposition methods, we determined that sex, age, and plasma levels of HDL cholesterol, apoA-I, and creatinine accounted for 38.6% of the variance in plasma EC-SOD levels and that additive genes accounted for 35% (P<0.00002). Multivariate analyses of plasma levels of EC-SOD, NO(x) (a measure of basal NO production), and apoA-I detected significant genetic correlations, indicating pleiotropy between EC-SOD and apoA-I (genetic correlation [rho(G)]=-0.45) and between NO(x) and apoA-I (rho(G)=0.58) but not between EC-SOD and NO(x). Genes shared by EC-SOD and apoA-I account for 20% of the genetic variance and, respectively, 7% and 9% of the phenotypic variance in both traits. Shared genes also account for >33% of the genetic variance and 5% and 15% of the respective phenotypic variance in NO(x) and apoA-I. In healthy individuals, over a third of the variance in EC-SOD plasma levels is due to the additive effects of genes. Some genes influence EC-SOD and apoA-I levels. The same is true of NO(x) and apoA-I but not of EC-SOD and NO(x). These patterns of pleiotropy can guide subsequent attempts to identify the genes and physiological mechanisms underlying them.