This paper reports on three studies of the changes in teacher effects on student learning and attitudes as a function of teacher practice. Two studies were performed in a tutorial context, the first with 4 tutors and 16 students, the second with 17 tutors and 68 students. The third study used small group instruction with 12 teachers and 288 students. In each study, the same teachers taught a short curriculum unit three or four times, teaching different students in each trial. The results of the studies showed marked individual differences among teachers in the extent to which their students learned more or less. Few teachers showed marked increases in student learning with practice. It was also shown that, on the average, student achievement decreased with teacher practice. Student attitudes toward the learning situation and toward themselves as learners changed in a curvilinear fashion. (Several graphs are included to illustrate the data.) (Author/RC)