The American College Testing (ACT) scores of New Mexico public high school graduates who applied for admission to New Mexico State University in 1981 were analyzed to determine the relationship between ACT scores and school size. The schools were divided into 6 categories according to enrollment: less than 100 students (15 applicants), 100-199 students (77 applicants), 200-499 students (175 applicants), 500-999 students (982 applicants), 1,000-1,999 students (1,539 applicants), and over 2,000 students (658 applicants). Analysis was performed for English, Math, Social Science, Natural Science, and combination scores. The smallest schools had the highest ACT scores in every case except for English, where they were second to the largest schools. Schools with 200 to 499 students scored lowest on all tests. In addition to showing significant differences in ACT scores among schools of different sizes, the study indicated that students from the majority of rural schools are weak in those areas measured by the ACT. These findings have important implications for those agencies providing assistance to the public schools in the state. High schools enrolling 100 to 500 students are in greatest need of improvement. (JHZ)