The study was designed to determine whether 16 aggressive mentally retarded adults could reduce their resting electromyographic (EMG) levels through relaxation training techniques. Ss were selected from a large state institution. Ss were divided into two groups--the experimental group received six training sessions which included whole body relaxation training followed by 5 minutes of EMG biofeedback training, and the control group which listened to music for the same amount of time. Following the recording of the first baseline EMG levels, Ss in the experimental condition participated in 5 minutes of whole body relaxation exercise and were asked to observe the investigator modeling the exercise. At the end of muscle relaxation exercises, Ss were asked to remain relaxed and to continue to breathe slowly and deeply. After training, EMG baselines were again recorded. Ss were also instructed in listening to the EMG instrument to monitor the relaxation in their muscles. The control group was attached to the biofeedback equipment but did not receive training or contingent EMG feedback. Results of the study demonstrated that retarded individuals were able to significantly lower their EMG levels as a result of muscle relaxation training. No carryover of training effects was found between sessions. Tables with statistical data are included. (SW)