Effects of abstaining temporarily from tobacco smoking were studied in a group of habitual smokers during a 15-day period, during which they smoked normally for the forst 5 days, refrained from smoking the next 5 days, and smoked again during the last 5 days. Results were evaluated against values obtained in a nonabstaining group of smokers. Adrenaline and noradrenaline excretion decreased, skin temperature increased, and hand steadiness was improved when the subjects stopped smoking. Submaximal, physical work tests were performed once each period. No changes occurred in perceived exertion for abstainers during work, in spite of a reduced heart rate. There were only minor differences between abstaining and smoking subjects with regard to performance in the cognitive tests. Irritation, depression, lack of concentration, sleep disturbances, anxiety, tension, and restlessness were frequently reported as abstinence symptoms. The results indicate a decrease in arousal level during abstinence.