Renal kallikrein is a component of the kallikrein-kinin-system (KKS). Kallikrein has been shown to cleave the precursor kininogen to release small kinins, which cause vasodilatation, increased diuresis and natriuresis. We have studied a normal restriction fragment length polymorphism (RFLP) at the renal kallikrein locus (KLK1), detectable with the restriction enzyme TaqI. In one series of 167 unrelated individuals we found a trend towards an association between genotypes in this polymorphism and level of diastolic blood pressure (DBP), but in two other series comprising 123 and 213 unrelated individuals, respectively, we found no suggestion of an association. Since the three series did not exhibit a consistent pattern of association between DBP levels and genotypes in this RFLP, we conclude that the association that appeared in one of the series was probably a chance event. There was no difference between genotypes in any of the three series, with respect to systolic blood pressure (SBP). In two series of, respectively, 157 and 120 complete monozygotic (MZ) twin pairs, there was no difference between genotypes with respect to within-pair variation in SBP or DBP. This indicates that normal KLK1 genes, expressed as variants in this RFLP, do not participate in the determination of the limits within which life-style factors may cause blood pressure (BP) changes. We conclude that neither "level gene" effects nor "variability gene" effects at the KLK1 locus are detectable with the polymorphism analyzed, in the Norwegian population.