In a field study, 24-h heart rate, physical activity, and cigarette consumption were continuously recorded, whereas resting heart rate, blood pressure, dietary intake, and subjective parameters were assessed six times per day. Smoking habitual and ultralight cigarettes for two days each was compared in a sample of 48 smokers, consisting of 24 office and 24 nonoffice workers of both sexes. Nonoffice workers smoked more and revealed higher respiratory CO and Fagerstrom index values, whereas other group differences were unrelated to smoking or its effects. Switching to ultralight cigarettes with four- to eightfold lower nicotine yields than the habitual cigarettes significantly decreased respiratory CO by 5 ppm, saliva cotinine by 30%, resting heart rate by 2.5 bpm, systolic blood pressure by 3.5 mmHg, and diastolic blood pressure by 3.0 mmHg, but increased fat intake by about 400 kJ, whereas activity and subjective well-being were not affected. Thus, the reduction in saliva cotinine was considerably smaller than the reduction in nicotine yield, and the effects on physiological parameters therefore were rather modest.