In ET-1+/− heterozygous mice, levels of ET-1 in plasma and lung tissue were lower than wild type, and the animals developed elevated blood pressure and mild hypertension rather than the fall in blood pressure that might have been expected (Kurihara et al., 1994) but explainable by ETB-mediated dilatation as the predominant action of ET-1 in the wild-type mice. This evidence concerns the gene EDN1 and Hypertension.