The spectrophotometric determination of various drugs having a carboxyl function is described. The method involves flow injection and specific detection of carboxylic acids based on the formation of 2-nitrophenyl-hydrazide derivatives mediated by a water-soluble carbodiimide. Using this system, the linear calibration range spanned three orders of magnitude from 125 pmol to 200 nmol and both hydrophilic and hydrophobic drugs could be determined easily at a sampling rate of 100 per hour. A number of drug preparations were also analysed with a coefficient of variation of less than 1%(n= 10).