The developmental phase of a program to train graduate teaching assistants (GTA's) attempted to identify the training requirements of GTA's and to experiment with a variety of training instruments and procedures. Volunteer GTA's (30) from nine different college departments participated in the 10-week program during which each GTA conducted his regularly assigned class in the Experimental Classroom Facility where each lesson was videotaped. After each class, the GTA viewed the tape, selected short segments illustrating classroom events of interest to him, and met regularly with five other GTA's and a training leader (acting as a resource person) to view and discuss the instructional problems in the selected videotape segments. Two other feedback methods were available to the GTA: Post Class Questionnaires from students and videotaped Student Debriefings during which four or five of his students discussed class videotapes with a member of the training staff. GTA's generally endorsed the program ranking skill improvement above information acquisition or attitude change. Four key principles characterize the procedures ultimately developed: (1) Exercise the classroom system as a whole; (2) Structure a learning environment in which participants can generate their own goals and determine their own conditions of practice; (3) Maximize objective feedback; (4) Provide the opportunity to analyze feedback in a nonthreatening group setting. (JS)