Decreased urinary kallikrein excretion has been shown to be related to hypertension. Kallikrein levels also have been shown to be determined primarily by genes, with 51% of the total variance being due to a single gene. However, there exists strong spouse-spouse correlation, indicating that common environment plays a significant role. This study used 69 pairs of monozygous twins to investigate possible dietary, biochemical, and anthropometric determinants of kallikrein that could result in this high spouse correlation. Urinary sodium and potassium excretion differences were significantly related to kallikrein differences, with urinary potassium having the strongest relationship (r = 0.46, Ρ = .0001). Urinary pH (r = 0.23, Ρ = .03) and systolic blood pressure (questionnaire that ascertains usual intake over the last 5 years. Kallikrein differences between monozygous twins were not explained by differences in nutrient intake as measured by this questionnaire. Therefore, urinary potassium and pH probably represent the more acute effects of recent dietary sodium and potassium intake on urinary kallikrein levels. Urinary potassium, pH, and systolic blood pressure differences explained 34% of the difference in kallikrein levels between monozygous twins. The significant difference in systolic blood pressure between twins, even aft;r = —0.25, Ρ = .03) differences were associated with urinary kallikrein excretion differences independently of urinary potassium. Information on nutrients was obtained from a dietary food frequency er controlling for electrolyte excretion differences suggests an additional unmeasured environmental variable that is associated with decreased kallikrein excretion and elevated blood pressure. Am J Hypertens 1993;6:226-233