Thirty five volunteer college women were divided into three groups to determine if heart rate could be conditioned instrumentally and lowered during exercise stress on the treadmill. The three groups were a) experimental group I, 15 subjects who received instrumental conditioning with visual feedback; b) instrumental group II, 9 subjects who received instrumental conditioning with no visual feedback; and c) control group, subjects who received no conditioning. All experimental subjects received ten days of instrumentally conditioned learning. The criterion of learning was the lowering of the heart rate 10% of the resting heart rate on three of the four trials presented each day for two consecutive days, while in the resting position. After the conditioning period, the conditioned stimulus for heart rate lowering was presented to all subjects in each of the three groups upon four levels of exercise stress: a) heart rate 100-120 bpm, b) 120-140 bpm; c) 140-160 bpm; d) 160-180 bpm. Analysis of the variance and Duncan's Multiple Range Test yielded significant differences between experimental group I and the control group at all levels, and between experimental group II and the control group at each level except 100-120 bpm. It was concluded that volitional control of heart rate may be successfully accomplished under exercise stress. Control of the heart rate in a resting state appears to transfer and facilitate heart-rate lowering under exercise stress. (Author)