1. The cardiovascular responses to sustained hand-grip contractions at 20, 30 and 50% maximal voluntary contraction (MVC) were measured in subjects who were engaged in treadmill walking at three different rates with oxygen intakes of 1.1, 1.7 and 2.8l./min. The increments in heart rate and blood pressure at tensions of 20 and 30% MVC were similar at all rates of walking, but the response to a contraction at 50% MVC was lower at the hardest work rate than at the two easier rates.2. When two or more muscle groups contracted at the same relative tension, the increments in heart rate and blood pressure were the same, whether they contracted separately or together.3. When two or more muscle groups contracted simultaneously at different relative tensions, the increments in heart rate and blood pressure were the same as when the muscle group, at the higher relative tension, contracted separately at that tension.4. The blood flow to a muscle engaged in sustained contraction was increased when a second muscle group contracted at a higher relative tension.