Based on clinical evidence, cryokinetics (alternating cold and exercise) is replacing heat modalities as the preferred therapy for rehabilitation of traumatic musculoskeletal injuries in athletes. Theories have been advanced to explain the clinical successes of cryokinetics, but little scientific data have been collected. Strain gauge plethysmography was used to measure blood flow to the ankle of 12 uninjured male subjects. A repeated measures design was utilized with each subject being tested under six experimental conditions: 1) heat packs, 2) cold packs, 3) control, 4) heat-exercise, 5) cold-exercise, 6) control-exercise. Exercise consisted of 5 three-minute bouts (3.5 mph) interspersed with heat, cold, or control throughout a 45-minute period. Non-exercise, heat and cold were administered for 25 minutes each, followed by 20 minutes without treatment. Instantaneous blood flow was measured regularly during non-exercise periods, estimated during exercise, and total flow was computed by integrating over the 45 minute treatment-post treatment period. Total flow (ml flow/100 ml tissue/min) was greater/p. less than .0002) during cold-exercise than during heat treatments. Contrary to some theories, there was neither cold-induced vasodilatation during, nor a reflex vasodilatation following, the 25-minute cold application. These data suggest that during cryokinetics, exercise causes the increased blood flow, and that cold applications function only to allow active motion in a painful joint.