Education students' attitudes towards mainstreaming are crucial to the future success of the movement toward inclusive education. This study describes the attitudes toward mainstreaming held by undergraduate education students and assesses attitude changes related to knowledge about handicapping conditions and the field of education as well as classroom teaching experience. The Attitudes Toward Mainstreaming Scale (ATMS) was administered to 233 undergraduate education majors; 110 students participated in a follow-up study. In general, education students favor the idea of mainstreaming and are willing to teach in regular classes those students whose handicaps do not inhibit their own learning or the learning of others. Cross-sectional and longitudinal data indicated that advanced students held more positive attitudes toward mainstreaming than beginning students and there was a trend toward reporting more favorable attitudes toward mainstreaming during professional preparation prior to student teaching. Following student teaching, however, there was a significant decline in the favorability of students' attitudes toward mainstreaming. Use of the ATMS and implications of the findings of the survey for teacher training are discussed. (Author)