A highly sensitive square-wave adsorptive anodic stripping voltammetric method was described for the determination of diflunisal in its formulations and human blood, utilizing a developed montmorillonite-Ca-modified carbon paste electrode (CPE). The peak current was significantly enhanced due to the strong adsorptive properties of montmorillonite-Ca clay. The optimal procedural parameters were frequency f = 80 Hz, scan increment Delta E (a) = 10 mV, pulse-amplitude Delta E (i) = 25 mV, and an accumulation potential E (acc) of 0.0 V versus Ag/AgCl/3M KCl in acetate buffer of pH 5.0 using 10% (w/w) MMT-Ca-modified CPE. The described method was successfully applied for assay of diflunisal in different pharmaceutical formulations (Doloban(A (R)), Dolozal(A (R)), and Maxipan(A (R)) tablets) with mean percentage recoveries of 98.72 +/- A 0.35, 99.24 +/- A 0.89, and 98.20 +/- A 1.38, respectively. Furthermore, the method was successfully applied for assay of diflunisal in spiked human serum without the necessity of sample pretreatment or time-consuming extraction prior to the analysis. Mean percentage recovery of diflunisal in human serum was 99.16 +/- A 1.03 with a limit of detection of 3.0 x 10(-9) M (0.75 ng mL(-1)). Due to this extremely low limit of detection, the proposed method was used to follow up the concentration of drug in blood samples of two male volunteers after oral administration of a single dose of Dolozal(A (R)), 500 mg tablet.