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We investigated the determinants of plasma renin activity (PRA) and plasma levels of angiotensin-coverting enzyme (pACE), including the effect of the D/I polymorphism of the angiotensin-converting enzyme (ACE) gene, in monozygotic (MZ) and dizygotic (DZ) twins.Sixty-nine pairs of twins underwent measurements of blood pressure, pACE and ACE D/I genotyping. In addition, in 30 pairs ambulatory blood pressure (ABP) monitoring was carried out. To ascertain twin's zygosity, some highly discriminating variable number of tandem repeats micro- and mini-satellite systems were analysed by polymerase chain reaction (PCR) followed by polyacrylamide gel electrophoresis and silver staining. The D/I polymorphism was assessed by PCR; pACE was measured in triplicate with a colorimetric assay, and PRA by a commercial kit. In DZ twins, identity by descent of the D/I alleles was examined by PCR amplification of a highly polymorphic simple sequence repeat at the human growth hormone gene.pACE levels were significantly (P < 0.01) higher in DD (9.27 ± 2.60 IU/I, mean ± SD) than in II (6.68 ± 3.0), with DI having intermediate levels (7.93 ± 2.7). No difference of PRA between different D/I genotypes was found. Twin data analysis showed a statistically significant heritability of pACE, but not of PRA. No differences between MZ and DZ twins in PRA, pACE and the relationship of the D/I genotype with pACE was found. Besides showing that the D/I genotype was the most important predictor of pACE, a multivariate analysis demonstrated that identity by descent of the D/I allele, as assessed by growth hormone (GH) genotyping, also significantly affected pACE.In this study of normotensive twins, pACE and not PRA showed significant heritability, the former being tightly associated with the D/l ACE gene polymorphism, and/or with a quantitative trait locus in linkage disequilibrium with it.