It is known that the diuretic activity of so-called loop diuretics is characterized by the ability of such compounds to inhibit the Na+, K+-cotransport system of the cell membrane. A good correlation has been demonstrated of the inhibitory activity of several furosemide- and ethacrynic acid-related compounds with their natriuretic activity in man. It was the aim of the study to examine, wether muzolimine has Na+, K+-cotransport inhibitory activity, and if so, if this occurs in a dose range which might be relevant for its natriuretic activity. For reasons of comparison, several other diuretics with high ceiling activity were studied. Cotransport inhibitory activities were compared with natriuretic activities in rats. Na+, K+-cotransport was measured by determination of furosemide-sensitive, ouabain-insensitive Na+-extrusion from Na+-loaded human red blood cells. Whereas the natriuretic activity of muzolimine is equal to that of furosemide in rats and superior in man, its cotransport inhibitory activity is merely 1/13 of that of furosemide. Additionally, only 50% inhibition is reached. The inhibitory activity of muzolimine, in contrast to ethacrynic acid, is not enhanced in the presence of equimolar cysteine. Muzolimine therefore appears to act via other mechanisms than furosemide-like diuretics. However, it cannot be excluded that muzolimine might act by an active metabolite.