SEVERE anxiety is invariably accompanied by a phylogenetically primitive reflex, which has been called the defence reaction, in preparation for either “fight” or “flight”. Although these behavioral responses are inappropriate in civilised society, defence reaction arousal occurs in expectation of an emergency. The cardiovascular system plays an important part in this reaction, and blood is redistributed from the viscera and skin to important structures such as skeletal muscle, the myocardium and the brain. Many of the changes can be observed in the arm, as there is an increase in heart rate (HR), forearm blood flow (FBF), blood pressure and cardiac output; accompanied by cutaneous vasoconstriction in the hand and a decrease in finger pulse amplitude. The increase in HR can be measured with a photo-electric transducer on the palmar surface of the index finger; a small lamp transilluminates the skin and a photo-electric cell records changes in the blood supply. Forearm blood$ow (FBF) The forearm, unlike the hand, is composed mainly of muscle, and in 1941 Wilkins and Eichna [l] showed that stressful mental arithmetic could produce a considerable increase in FBF. This is mediated by nervous and humoral (adrenaline) mechanisms. There are sympathetic cholinergic vasodilator fibres, which are not active at rest, but appear to be highly specific in their response to emotional stress [2]. FBF can be measured with a water-filled venous occlusion plethysmograph, or a mercury-inrubber strain-gauge (Kelly [3]). If the venous return from the forearm is completely blocked for a few seconds, without interference to the arterial inflow, the resulting increase in volume per unit time is equal to the blood flow. The rate of blood flow (ml/100 ml of forearmlmin) can be calculated from the gradient of the plethysmographic tracings [2, 31. A gentle gradient indicates a low rate of flow, as illustrated by the normal control Fig. 1. In contrast, a patient suffering from chronic anxiety who was highly aroused even in the resting state, had steep gradients (5.1 and 5.2) which were more than twice as great as those of the control (Fig. 1). When stressed with mental arithmetic the values of both the control and the patient increased to about 8 (ml/100 ml/min), so the percentage increase was less in the patient because of his high resting FBF. In calculating the results, the mean of the 3 lowest values taken at minute intervals during a 15 min rest period, was calculated and called “basal”, as this was thought to reflect the period of greatest mental relaxation; the peak value, recorded at 30 set intervals during 29 min stressful mental arithmetic, was called the “stress” FBF. The “basal” FBF has been found to be a good index of physiological arousal in psychiatric patients [4, 7, 9, 111. Sweat gland activity Anxious patients have cold vasoconstricted hands which are usually moist due to increased sweating. Sweat gland activity can be measured indirectly by the electrical