AbstractAn assay was developed for benzthiazide in plasma, urine and feces, using high performance liquid chromatography (HPLC). A reverse‐phase column was employed, with quantitation at 280 nm, using polythiazide as an internal standard. In three of four human subjects who received a 50 mg benzthiazide tablet the plasma concentrations were below the 10 ng ml−1sensitivity limit of the assay, and the urinary recovery averaged less than one per cent of the dose. One subject received a 50 mg dose as both a tablet and a solution; the urinary recoveries for these two doses were 1.7 and 10.4 per cent, respectively. Fecal samples, obtained from two subjects who received 50 mg tablets, were estimated to contain approximately 80 per cent of the administered d