The conversion of 11-deoxycorticosterone (DOC) to aldosterone is catalyzed by a single enzyme, termed P450c11AS, which has 11 beta-hydroxylase, 18-hydroxylase and 18-oxidase activities. The normotensive Dahl salt-resistant (R) rat has two mutation in P450c11AS that increase its aldosterone synthase activity. If such a mutation were to occur in human patients the predicted phenotype would be low-renin hypertension with elevated ratios of plasma aldosterone to plasma renin activity. Before searching for P450c11AS mutations in such patients we sought to determine if mutations in human P450c11AS could increase enzymatic activity in a fashion analogous to the Dahl R rat. We used site-directed mutagenesis of the human P450c11AS cDNA to create the mutants Glu 136-->Asp, Lys 251-->Arg and the combination of the two; these mutations correspond to those seen in the Dahl R rat. Cells transfected with these mutant human P450c11AS sequences could convert [14C]DOC to corticosterone, 18OH-corticosterone, and aldosterone. In particular the Lys 251-->Arg mutant produced 4 times as much 18OH-corticosterone and 50-80% more aldosterone than the wild type. These data show that mutations of human P450c11AS can increase enzymatic activity, suggesting that such mutations could, in theory, be the basis of some forms of human low-renin hypertension.