After rinsing with a fluoride solution, the amounts of fluoride taken up by small pieces of dentine placed strategically about the mouth varied considerably from site to site in the oral cavity. The pattern of fluoride availability in this subject's mouth was in line with previous findings about fluoride distribution in his labial sulcus. The present results therefore suggest by analogy that considerable variations in fluoride clearance and concentration must exist throughout the oral cavity. These previous studies of fluoride clearance from the labial sulcus also showed that there are marked differences between individuals. The present findings, emerging from a study of 1 subject, therefore imply a need for more information about the variations in the availability, concentration and clearance of fluoride and, by extrapolation, of other substances throughout the mouths of different individuals.