A study investigated whether teachers who receive training and then implement more effective instructional practices change in the reasons they offer for their effectiveness. Comparisons were made between 46 intermediate and high school teachers in a large urban school system who were trained in and implemented mastery learning techniques, and 50 similar teachers who could not be included in the training. A questionnaire was administered to both groups before and after the training. Results of a multivariate analysis of variance showed that the mastery learning teachers attached increased importance to teaching practices and behaviors in explaining their effectiveness after they had implemented these techniques. Furthermore, there was a significant reduction in the importance mastery learning teachers attached to personality factors. The implications of these findings for staff development are discussed. (Author/JMK)