Using the recollection micropuncture technique it was demonstrated that the intragastric administration of triamterene significantly reduced the fraction of filtered potassium remaining (from 27 to 17%) and enhanced the fraction of filtered sodium remaining (from 3.6 to 7.9%) at the end of the accessible distal tubule in 7 rats. Fractional delivery of water, sodium and potassium to the distal tubule was not significantly altered. Urinary potassium excretion was reduced (from 17 to 7%). Similar experiments in 6 control rats disclosed no significant changes in the foregoing variables. In these experiments the principal effect of triamterene in vivo was an inhibition of potassium secretion in the distal tubule. An ostensible inhibitory effect on sodium reabsorption in the distal tubule was not unequivocally established.