This investigation analyzed sex-related and age-related differences on spatial visualization tasks, reasoning tasks, and mathematical problem-solving tasks; and the effects instruction in visual thinking would have on these differences. Students in fifth grade, ninth grade, and in college, were tested before and after twelve weeks of instruction on seeing, imagining, and drawing two-and three-dimensional figures. Four tests were used to measure spatial visualization: the Mental Rotations Tests, and three sections of the Kit of Reference Tests for Cognitive Factors: Punched Holes Test, Form Board Test, and Hidden Figures Test. There were two measures of reasoning: Nonsense Syllogisms Test, and Reasoning Test; and a newly-developed problem-solving test. Results indicated that problem-solving correlated significantly with spatial skills, reasoning skills, and degree of visuality. Older students performed significantly better on all tests. Males performed better on spatial tasks while females performed better on reasoning tasks; with increasing age, the gap between sexes widened on spatial and reasoning tasks, but not on problem-solving tasks. Instruction affected spatial and reasoning ability, but not problem-solving performance or degree of visuality. (The problem solving test is appended). (CP)