Richardson's ground squirrels of both sexes were fed a 2% cholesterol-enriched rat chow diet for various intervals from 6 hours through 20 weeks. Bile was withdrawn from the organs and analyzed biochemically to obtain a lithogenic index. The remaining luminal contents were rinsed from the resected organs and examined by light and scanning electron microscopy. Gallbladder tissue was fixed and examined in a similar fashion to observe the luminal aspect of the epithelial sheet. Scanning electron microscopy revealed increased mucus secretion before precipitation of rhomboidal cholesterol monohydrate crystals from the lithogenic bile. The crystals grew by appositional layering into microliths and then by aggregation into mulberry-shaped stones. Mucus hypersecretion continued to be a common epithelial observation throughout the study. Five days after introduction of the diet, a thick discontinuous sludge-like layer was observed overlying large regions of the epithelial sheet. In later stages, this material was observed embedding cholesterol crystals into the surface aspect of the stone. By 20 weeks many stones were approximately 2 mm in diameter and were accompanied by large numbers of various sized concrements. The nature of the cholesterol crystals and stone formation pattern closely resembled that reported for humans. The ground squirrel fulfills all criteria required to be regarded as an excellent animal model and is well suited for continued study into the role of the epithelial cell during cholesterol cholelithiasis.