Previous research in locus of control (LC) suggested the hypothesis that internal Ss should perform better under mastery than under traditional assessment procedures, while the reverse should be true of externals. Two experiments were conducted using undergraduate and graduate Ss. Neither the LC nor the assessment procedure main effects were significant in either study, and no interaction was found with the undergraduates. With graduate Ss there was a significant interaction opposite in direction to expectations. Ss overwhelmingly preferred the mastery procedures. These results are harmful to the construct validity of the I-E Scale (Rotter, 1966) and supportive of the mastery learning approach. (Author)