• Unmedicated cellulose polymer (UCP) inserts exhibited marked superiority over six commercial artificial tear substitutes in enhancing and maintaining precorneal tear film stability as measured by tear film breakup time (BUT) in rabbits. The application of UCP inserts into one eye also resulted in an appreciable elevation of the BUT in the contralateral untreated eye. Sham inserts (polyethylene pieces) equal in size to or larger than UCP inserts lacked similar effects, which suggests that the latter actions of UCP inserts are most likely due to a slow release of polymer into the tear film rather than to induction of reflex responses associated with insertion of an inert foreign body into the cul-de-sac. The results indicate that UCP inserts, by virtue of their ability to maintain the integrity of the precorneal tear film over extended periods of time, may represent an important advance in the treatment of dry-eye diseases.