The application of the ratio spectra derivative spectrophotometry and high-performance liquid chromatography (HPLC) to the simultaneous determination of paracetamol (PAR) and methocarbamol (MET) in combined pharmaceutical tablets is presented. The spectrophotometric procedure is based on the use of the first derivative of the ratio spectra obtained by dividing the absorbtion spectrum of the binary mixtures by a standard spectrum of one of the compounds. The first derivative amplitudes were measured at 243.0 and 230.3 nm for the assay of PAR and MET, respectively. Calibration graphs were established for 2-30 mug/ml for PAR and 2-36 mug/ml for MET in binary mixture. The detection limits for PAR and MET were found 0.097 and 0.079 mug/ml, respectively; while the quantification limits were 0.573 mug/ml for PAR and 1.717 mug/ml for MET. For the HPLC procedure, a reversed-phase column with a mobile phase of methanol-water (60.40, v/v), was used to separate both compounds with a detection of 274.0 nm. Linearity was obtained in the concentration range of 2-300 and 1.5-375 mug/ml for PAR and MET, respectively. The detection and quantification limits were found to be 0.42 and 1.4 mug/ml for PAR and 0.36 and 1.2 mug/ml for MET, respectively. The relative standard deviations were found to be less than 0.52%. indicating reasonable repeatibility of both methods. The proposed methods were successfully applied to the determination of these drugs in commercial tablets. (C) 2001 Elsevier Science B.V. All rights reserved.