The rate of fluorine uptake from sodium monofluorophosphate by four layers of sound bovine enamel of monofluorophosphate enamel was determined.The rate differed depending on the concentration in the treatment solution and the depth of enamel analyzed.When the enamel was treated with monofluorophosphate concentrations (10 ppm F) the rate was directly proportional to time at all depths analyzed .With higher concentrations (100, 1,000 ppm F) the rate was directly proportional to time only in deep enamel and then only after the first hour.