IN AN earlier paper (Mueller, 1962) it was reported that the diuretics mercuhydrin, cardalin and acetazolamide interfered with egg shell formation. The present report deals with the aldosterone antagonist spironolactone which causes diuresis in mammals.Reviews on aldosterone antagonists have been prepared by Bartter (1960) and DeStevens (1963). It is generally agreed that spironolactone causes diuresis by blocking the urinary electrolyte effects of aldosterone and other mineralocorticoids. Thus, spironolactone causes an increase in sodium and a decrease in potassium excretion. All evidence indicates that spironolactone acts at the kidney level; it does not inhibit aldosterone formation at normal dosages. The effectiveness of spironolactone in mammals is limited. The changes in electrolyte balance are usually observed only in the presence of hyperaldosteronism (Salassa et al., 1958) or in adrenalectomized animals (Kagawa, 1960). In intact rats and dogs Kagawa (1960) found no changes in urine volume and sodium excretion after spironolactone doses …