SummaryThe effects of mental arithmetic on heart rate, arterial blood pressure, hand blood flow and resistance to hand blood flow have been tested. Heart rate and mean arterial blood pressure always increased. In cool conditions, when resting hand blood flow was less than 4·9 ml/100 ml/min, mental arithmetic caused no change in resistance to hand blood flow. Under warm conditions, when resting hand blood flow was greater than 4·9 ml/100 ml/min, mental arithmetic caused a sustained rise in resistance to blood flow in the hand.