The performance of 2,000 academically talented seventh-grade students on the American College Testing Program (ACT) Assessment was investigated in comparison with 2,000 college-bound 12th-grade examinees. Data from the February 1988 and February 1989 administrations of the ACT were used. Differential test performance and item performance were examined based on one form of the ACT, and the results were cross-validated on a different form. Although the 12th-graders outperformed the seventh-graders on all tests, the differences were greatest on the mathematics and social studies reading tests. In addition, systematic performance differences between the two groups of students were found at the item level. The nature and implications of these differences are discussed. Four tables present study data. Three figures illustrate six types of test items. A 7-item list of references is included. (Author/SLD)