Fourth derivative UV-spectrophotometric and high-performance liquid chromatographic (HPLC) methods for simultaneous determination of fosinopril and hydrochlorothiazide in tablets have been developed. Standard solutions were measured at zero crossing wavelengths of 217.7 and 227.9 nm for fosinopril and hydrochlorothiazide, respectively, by fourth derivative spectrophotometric method. Calibration curves were constructed by plotting d(4)A/d lambda (4) values at selected wavelengths against concentrations. HPLC analyses were carried out on C-18 column with gradient elution by using 10 mM H3PO4 and CH3CN as mobile phase. Benazepril was used as internal standard and the substances were detected at 215 nm. Commercially available tablets containing 20 mg fosinopril and 12.5 mg hydrochlorothiazide were analysed by fourth derivative spectrophotometric and HPLC methods. The results were compared statistically at 95% confidence level with each other. There was no significant difference between the mean percentage recoveries and precision of the two methods. (C) 2001 Elsevier Science B.V. All rights reserved.