The electrooptical property of clay particles dispersed in aqueous media was studied with a newly constructed reversing-pulse generator, in which a field-effect transistor (abbreviated as MOS-FET) rather than a vacuum tube was used as the switching element. This pulse generator has the following characteristics : (1) a stable output voltage in the 5-300 V range, (2) a widely variable output pulse duration in the 10 mu s-1.2 s range, and (3) a very short time constant of electric pulse (generally 150 ns for buildup and reverse and 50 ns for decay). Moreover, this generator could deliver a single rectangular pulse and/or a reversing pulse to high ionic strength samples, e.g,, 0.2 mol dm(-3) NaCl of MgCl2, because of a high electric current resistance of MOS-FET. By using this new reversing-pulse generator, a study was carried out on the effect of the ionic strength, valence of electrolyte, and particle concentration on the electrooptical and hydrodynamic properties of montmorillonite particles dispersed in aqueous media. The analysis of measured reversing-pulse electric birefringence signals of Na-montmorilonite showed that the particle possesses no permanent dipole moment but that an ion polarization makes a large contribution to the field orientation, The sodium ion, trapped on the cation-exchangeable site, may be responsible for this ion polarization; the Na-montmorillonite particles also form aggregates of card house structure at increased concentrations of added sodium ions. In contrast to the Na-montmorillonite particle, reversing-pulse electric birefringence signals of Mg-montmorillonite suspensions showed longer average rotational relaxation times. The signals could not be analyzed theoretically because of larger aggregates of book house structure, The optical anisotropy of the clay particle was increased with an increase in Mg2+ concentration.