Several tests of cation concentration and transport are being studied among members of large Utah pedigrees as part of a study of the genetic and environmental determinants of essential hypertension. Corrected urinary sodium excretion and plasma sodium concentration correlated well in spouses and siblings (r = 0.21-0.54, p less than 0.001), suggesting the effects of shared family environment (e.g., sodium intake). Intraerythrocytic sodium concentration and sodium-lithium countertransport showed no significant correlation in spouses and very significant correlations between siblings and between parents and offspring (r = 0.34-0.58, p less than 0.001), suggesting mostly genetic determination. Using maximum likelihood tests of different genetic models, both sodium-lithium countertransport and intraerythrocytic sodium showed predominantly polygenic determination (H2 = 70%) and some possible major gene determinants (H2 = 18-25%) for a total heritability of 89 to 95% for these characteristics. These data suggest both genes and shared family environment contribute to the familiality of cation tests. They also illustrate the need and utility of quantitative methods for objective analysis of pedigree data.