A novel use of periodic functions, called the periodic procedure, was recently introduced to make practical the use of certain physical measurement ideas in psychology. This paper reports a successful attempt to apply the periodic procedure to an important psychological measurement problem, the measurement of aptitude test item difficulties. The difficulties of items on a subtest of the Scholastic Aptitude Test were measured with both the periodic procedure and with an item characteristic curve theory procedure. The principal finding was a close agreement between the two methods. This close agreement was particularly striking because different samples (from the same test administration) were used by the two methods. The statistical development of the periodic procedure is presented along with an example of its use. (Author/JKS)