The cataractogenic effects of the microwave oven on rat eyes were investigated histologically. Twenty-one adult Wistar-Albino rats (2-2.5 months old) were divided into three groups (n = 7): a control group (sham-exposed) and two experimental groups. The experimental rats were confined in special cages and placed next to the closed door of a microwave oven. The first experimental group was given 15 minutes of daily exposure and the second. 30 minutes, for 1 month. Biomicroscopic examination detected no pathological damage to the lens in the experimental rats. Histologically, there was a lens of single-layered epithelium in the control group. Tn the first experimental group. there was slight pleomorphism in the superficial epithelial cell contours and vacuolizations in the lens fibers: the second experimental group, had pronounced pleomorphism and pyknosis of the nuclei in the superficial epithelium. Some of the superficial epithelial cells had disappeared and were histologically observed as acellular areas, Single-layered epithelium became disorganized and formed multilayered epithelial groups in the superficial epithelium. Vacuolization was more prominent in this group. Personal exposure from microwave ovens is generally minimal because of the rapid decrease in power density with distance. Microwave oven users do not normally stand as close to the oven as the rats in our study were placed: therefore, it is difficult to suggest that microwave ovens always have cataractogenic effects on human eyes. (C) 1997 Japanese Ophthalmological Society.