School size, age and sex of students as related to scores on the six Piagetian Developmental Thought Processes Tasks were investigated. Five hundred seventy-four students from seventh through twelfth grades were randomly selected from 25 different schools classified as small, medium, or large. Data were treated through factorial analysis of variance and the Scheffe technique. Findings showed that school size, age and sex of respondent influenced the development of attainment of formal operational thought, with the age variable being the most significant influence and the school size, the least. Boys did better than girls on the tasks. Smaller school size was more influential on the less complicated tasks and larger schools were influential on the more complicated tasks. (Author/DT)