During the Spring of 1989, the North Carolina Department of Public Instruction field-tested a geometry proof performance assessment as a component of the High School End-of-Course Testing Program (EoC). The existing geometry EoC test consisted only of multiple-choice items. The performance assessment was added to the multiple-choice component to form a more "authentic" assessment of student performance. Focus was on determining the reliability of the scoring process and the cost of adding a performance assessment to the existing geometry test. The sample for the proofs and the comprehensive geometry multiple-choice test was over 43,000 students, who comprised the entire statewide enrollment in North Carolina in geometry classes. The separate 32-item proofs field test was administered to 875 students. A high degree of consistency was found between the ratings assigned by two readers when perfect and adjacent agreement findings were analyzed. The estimated cost of conducting the performance assessment was 3.00 dollars per student (2.44 dollars for school year 1990). The educational significance of using this process for developing authentic assessment strategies is discussed. Eight tables contain study data. Appendices provide North Carolina geometry goals/objectives; rated samples of geometry proofs and the score scale; percentage of students receiving each score on the 1988-89 Geometry Proof Field Test; statistics for proof scores and grades and geometry proof focused-holistic score scale distribution; a summary of teacher assigned proof grades and geometry proof scores; sample school system disaggregate report; and teacher survey data. (SLD)