A drug interaction study in the chimpanzee by using indomethacin and hydrochlorothiazide has shown conclusively that the diuretic and saluretic properties of hydrochlorothiazide were not compromised by indomethacin. This was true whether hydrochlorothiazide or indomethacin was administered first. The renal clearance of hydrochlorothiazide was not influenced by indomethacin nor was the renal clearance of indomethacin significantly altered by hydrochlorothiazide. Indomethacin alone caused a small but significant increase in sodium, potassium and chloride excretion and in Curate/glomerular filtration rate. In control experiments with placebo, potassium excretion was also significantly increased. The implications of these observations remain obscure. In all experiments utilizing hydrochlorothiazide, the well known renal effects of this agent were clearly evident under these experimental conditions.