An experimental investigation is described of the electrokinetic properties of nitrofurantoin. The experiments are based on measurements of electrophoretic mobility in suspensions of this drug and the calculation of its zeta potential (zeta) following two different procedures. A simple mechanism is proposed for explaining the sign of the surface charge of nitrofurantoin and its increase with the pH of the medium. The effect on zeta of changes in the concentration of NaCl, CaCl2, or AlCl3 is studied for different pH values of the suspensions. The behavior of zeta in the presence of NaCl shows evidence of Cl- adsorption on the particles, especially at low electrolyte concentrations. The AlCl3 salt was capable of reversing the sign of the surface charge, changing it to positive when a given, pH-dependent concentration was added to the suspension. The effect of this salt on zeta for different pH values was probably due to the hydrolysis of the cation Al3+. A significant increase of the negative surface charge was observed when a small amount (0.1%) of a synthetic polymer, Carbopol 934, was present in NaCl or AlCl3 systems. This appears to indicate that the large polymer molecules adsorb on nitrofurantoin particles as part of their stabilizing mechanism.