A SERIES OF RESEARCH PROGRAMS CONCERNED WITH THE EFFECTS OF SYSTEMATIC VARIATIONS OF CERTAIN CONDITIONS RELATED TO LEARNING HAS BEEN CONDUCTED. PROJECTS PREVIOUSLY REPORTED INCLUDED TWO INVESTIGATIONS OF REINFORCEMENT AND SUBJECT VARIABLES AND ONE ON THE STUDY OF PRACTICE AND SUBJECT VARIABLES. THE NATURE OF THE OVERALL PROGRAM WAS DISCUSSED IN DETAIL IN AN EARLIER REPORT (BLAKE, 1961). THIS INVESTIGATION, THE FOURTH OF THE SERIES, DEALT WITH THE EFFECTS OF SYSTEMATIC VARIATIONS IN TASK CONDITIONS ON THE LEARNING PERFORMANCE OF INTELLECTUALLY RETARDED, INTELLECTUALLY NORMAL, AND INTELLECTUALLY SUPERIOR STUDENTS. THE SUBJECT POPULATION CONSISTED OF STUDENTS REPRESENTING DIFFERENT CHRONOLOGICAL AND MENTAL AGES. THE SUBJECTS WERE ADMINISTERED TASKS IN WHICH THE CHARACTERISTICS OF THE MATERIALS AND PROCEDURES WERE SPECIFIED BY SEVERAL OVERLAPPING TASK CONDITIONS--TYPE OF MATERIAL, RESPONSE, LEVEL OF LEARNING, AND ABSTRACTNESS. THE CRITERION MEASURE WAS THE NUMBER OF ITEMS WHICH THE SUBJECT COMPLETED SUCCESSFULLY ON TESTS AFTER A SPECIFIED NUMBER OF TASK TRIALS. IT WAS CONCLUDED THAT TASK COMPLEXITY, ABSTRACTNESS LEVEL, TASK LENGTH, METHOD OF APPROACH, AND TYPE OF PRESENTATION AFFECT LEARNING. LIMITATIONS TO THE FINDINGS ARE DISCUSSED. (JC)