THE PROBLEM OF THE RESEARCH WAS TO STUDY THE RELATIONSHIP BETWEEN PERCEPTION AND COGNITION FACTORS IN THE GROWING CHILD. A CENTRAL TASK WAS TO DEVELOP AN INSTRUMENT FOR ASSESSING THE DEVELOPMENT OF SIZE CONSTANCY IN A PROCEDURE IN WHICH THE EFFECTS OF COGNITIVE ACTIVITY COULD BE EITHER CONTROLLED OR EVALUATED. USING A NUMBER OF CONSIDERATIONS, THE RESEARCHERS PLACED A STANDARD STIMULUS AT 14 FEET FROM THE SUBJECT AND A COMPARISON STIMULUS AT 4 FEET FROM THE SUBJECT. AS STANDARD STIMULI, POSITIONS NUMBERING FOUR AND SIX IN A NINE STIMULUS SERIES WERE SELECTED. TWO DIFFERENT STANDARDS WERE SELECTED SO AS TO COMPARE THE EFFECT OF SIZE ON POSITION-IN-THE-SERIES OF THE STANDARD ON JUDGMENTS OF THE COMPARISON STIMULI. PIAGET'S CONCENTRIC METHOD AS A VERSION OF CONSTANT STIMULI WAS USED. SIZE OF FEASIBLE APPARATUS TO HANDLE NINE THREE-DIMENSIONAL STIMULI CONTRIBUTED TO A DECISION ABOUT THE SIZE OF THE STIMULI. BOXES SERVED AS THE STIMULI. SLIDES WERE PREPARED AS PART OF THE STUDY OF THE COMPARISON BETWEEN TWO-VERSUS THREE-DIMENSIONAL STIMULI. SIGNIFICANT DIFFERENCES WERE FOUND BETWEEN TWO-DIMENSIONAL STIMULI AND THREE-DIMENSIONAL STIMULI WITH BOTH WEIGHTED AND SIMPLE SCORES. THE DATA WERE OBTAINED FROM 13 ADULTS AND ONLY ONE CHILD. (JC)