The effect of a series of diuretically active substituted 3-aminobenzoic acid derivatives and related compounds was investigated on a cyclic AMP-activated Na+-K+ cotransport system in avian erythrocytes. A good correlation between the diuretic potency of this class of "loop" diuretics in the dog and their inhibition of cation cotransport in turkey erythrocytes was found. Selected thiazide-type diuretics were found to be ineffective. The most active compound tested (3-benzylamino-4-phenylthio-5-sulfamoylbenzoic acid) had an effective dose50 of 4.6 x 10(-8) M in the avian system, and was about 5 times more potent than bumetanide and 500 times more potent than furosemide in this regard. The diuretics appear to interact directly with the cation transport system itself, and not with some antecedent step in the sequence of events from intracellular cAMP accumulation to stimulation of transport. The compounds tested did not appear to compete at Na+- or K+-binding sites on the transport system. The similarity in the structure-activity relationship of these agents in the avian erythrocyte and the kidney suggests that the avian erythrocyte may be a useful model for analysis both of the diuretic-sensitive transport system of the mammalian kidney, and of the molecular mechanism of loop diuretic action.