The renal glomerular mesangium in male, Swiss Webster mice, fed a diet containing 2.5 percent griseofulvin (GF), showed hypercellularity and mesangial thickening within three weeks. These changes gradually increased, peaking in prominence at 11 to 16 weeks and persisted up to 22 weeks. The tubular epithelium showed minimal light microscopic changes. There was focal hyperplasia of parietal cells of the Bowman's capsule. Ultrastructurally, the changes in the mesangial region of the glomeruli included increase in mesangial cells and mesangial matrix. The renal tubular epithelial cells showed dilatation of mitochondria and smooth endoplasmic reticulum. These changes, in addition to diverse cellular alterations in other organs previously reported, constitute the morphologic features of GF-induced renal disease.