In a double-blind placebo-controlled investigation, 14 healthy adult male volunteers were studied to assess and compare the urinary effects of acute single doses of two antihypertensive formulations containing: hydroflumethiazide 50 mg, Rauwolfia serpentina 50 mg and potassium chloride 625 mg (HFRK) (Rautrax-50; Squibb Laboratories); and clopamide 5 mg, reserpine 0.1 mg and dihydroergocristine 0.5 mg (CRE) (Brinerdin; Sandoz Products). Both significantly increased mean 24-hour urinary outputs of fluid, Cl-, Na+, K+ and Mg2+. CRE increased creatinine and decreased Ca2+ output. After dosing, times to maximal urinary flow of fluid, Cl-, Na+, Ca2+ and creatinine were shortened by HFRK and CRE and those corresponding to K+, Mg2+, phosphate and urate were unaffected. Both formulations thus acted mainly through their diuretic constituents.