When measured by a specific radio-assay method which excludes drug interference, plasma 17alpha-hydroxyprogesterone levels were found to be similar in spironolactone treated hypertensives, in untreated hypertensives, and in normotensive volunteers. No in vivo evidence for a significant inhibitory effect of spironolactone on adrenal 11beta, 17alpha and 21 hydroxylases was uncovered.