A procedure for simultaneous determination of metoprolol and hydrochlorothiazide in tablets by employing derivative spectrophotometry, "zero-crossing" method was developed. The third order derivative absorption spectra at lambda approximately 281 nm were used for metoprolol and the first order derivative spectra at lambda approximately 282 nm were used for hydrochlorothiazide. No interferences were found between both determined constituents and those of matrix. A good accuracy and precision of simultaneous determination of metoprolol and hydrochlorothiazide were confirmed by statistical analysis. The recovery of individual constituents under established conditions is very high and ranges from 98.79% to 99.39%. Linearity is maintained within a wide concentration range from 100.0 microg mL(-1) to 300.0 microg mL(-1) and from 12.5 microg mL(-1) to 37.5 microg mL(-1) for metoprolol and hydrochlorothiazide, respectively. The detection limit is 5.0 microg mL(-1) for metoprolol and 1.5 microg mL(-1) for hydrochlorothiazide. The corresponding quantitation limits are 15.0 microg mL(-1) for metoprolol and 4.5 microg mL(-1) for hydrochlorothiazide.