A method is presented which allows the average daily rate of enamel apposition to be estimated based on measurements of the length of prisms in longitudinal ground sections, and on available data for the time of formation of the tooth crown. The method was applied on 21 sections of human maxillary premolars. The reproducibility of the method was satisfactory, the error of measurement being in the order of 3.8%. After corrections having been made for the deviation of prisms from a radial direction, the average daily rate of enamel apposition along the prisms for any maxillary premolar was estimated to be within a range of from 2.0 to 6.1 microns, while a more probable rate would be 2.7-4.6 microns. These results are in agreement with experimental data, and are also compatible with the observed cross-striation prism periodicity. The present study supports the theory that there exists a relationship between the prism periodicity and a 24-h rhythm in enamel apposition in human teeth.