A zero-crossing first-derivative spectrophotometric method for the determination of piroxicam and its major metabolite 5-hydroxypiroxicam (5-HP) in human plasma is described. This technique permits the quantification of compounds with closely overlapping spectral bands without any separation step. The method consists of direct extraction of the less-ionised forms of piroxicam and 5-hydroxypiroxicam with pure diethyl ether. First derivative values at 343.5 and 332.5 nm for piroxicam and 5-HP, respectively, were obtained. The absolute recovery of the method was found to be 89.4% for piroxicam and 90.3% for 5-HP. Calibration graphs are linear (r better than 0.9998), with zero-intercept, in the concentration range 0.5-12.0 micrograms ml-1 for both compounds. The limits of quantification attained according to the IUPAC definition were 0.29 and 0.27 micrograms ml-1 for piroxicam and 5-HP, respectively. The results obtained by the proposed method were in good agreement with those found by the high-performance liquid chromatographic method (HPLC).