The diuretic, natriuretic and antikaliuretic properties of several straight-chain and branched-chain acidic triamterene derivatives were investigated in male Wistar rats. Among the straight-chain compounds the ether of hydroxytriamterene with five C atoms in the side chain (S3) revealed the maximum effects. The iso-compounds (derivatives of 4-carboxymethoxy triamterene S1) showed much better water solubility than the n-acids, but they increased the natriuresis only slightly and had no effect on kaliuresis when they were injected intravenously alone; whereas the isocompound Si4 showed potassium retaining properties when it was administered together with furosemide. From these results it can be suggested that the negative charge in the side chain has to be in a distinct distance from the pteridine ring to produce antikaliuretic effects.