Summary Young dairy calves equipped with rumen cannulae were fed on milk and housed on elevated wood platforms. Various materials such as plastic sponges, cellulose sponges, a purified diet, solutions of the sodium salts of acetic, propionic, butyric, and lactic acids were introduced through the cannulae for periods up to 12 wk. The presence of plastic sponges alone did not elicit development of the rumen papillae. The rumens of calves receiving either the purified diet or the fatty acid solutions, with or without sponges, showed marked development. These data confirm the view that end-products of rumen fermentation rather than the coarse nature of the feed are the stimuli for the development of rumen papillae.