This research investigated developmental shifts in the character of children's tool using activity in the domain of scale drawing. Fifty-five children from three grade levels (grades 3, 5, and 7) were individually interviewed as they participated in both enlarging and reducing a one-dimensional object, the letter "F," to a scale of four. Children participated in the enlarging/reducing activity under different tool conditions: once using a ruler, once using graph paper, and once using a moveable replica of the letter F. Three principal features of children's activities were analyzed: (1) the accuracy of children's constructions; (2) the ways children incorporated the tool into their problem solving; and (3) children's scale-linked understandings. Statistical analysis revealed that children's accuracy, their tool-using activity, and their scale understandings shifted with age, and these shifts interacted with the difficulty of the scale task (enlarging vs. reducing the F) and tool condition (replica vs. graph paper vs. ruler). Results support L. Vygotsky's work on tool use illustrating the development of sign operations, but his analysis is extended to incorporate children's conceptual understandings in the development process. An appendix contains three tables of means and standard deviations of study variables. (Contains 5 tables, 10 figures, and 53 references.) (Author/SLD)