The dose ratio approach was used to define the steady-state relative potency of the competitive mineralocorticoid antagonists prorenoate potassium and spironolactone in six healthy male subjects using fludrocortisone as mineralocorticoid agonist. Log fludrocortisone dose-response relationships in the presence or absence of antagonists did not differ from linearity and parallelism, supporting the theoretical basis of the method. Urinary sodium and plasma potassium responses appeared to behave according to the law of mass action, which made possible estimation of the potency of prorenoate relative to spironolactone on a weight basis-4.2:1 (95% C.L. 2.7-6.9:1) and 2.68:1 (95% C.L. 0.71-6.57:1, respectively. The steady-state relative potency for sodium excretion was greater than previously estimated after single doses. Mass action theory could not explain the urinary potassium and log 10 Na/K responses to repeated doses of spironolactone, precluding valid estimation of relative potency for these variables and suggesting that the latter response alone is an unreliable index of overall renal antimineralocorticoid activity.