1. The breathing patterns of ten habitual smokers were monitored in the semi-recumbent position with respiratory inductive plethysmography before and after smoking cigarettes. The subjects smoked a high-tar-content (HTC) and a low-tar-content (LTC) cigarette. The mean (± sd ) values and frequency histograms of minute ventilation , tidal volume ( V T ), frequency ( f R ), inspiratory time ( T I ), fractional inspiratory time ( T I /T TOT ) and mean inspiratory flow ( V T /T I ) during baseline were compared with the values during and after smoking. 2. On a separate occasion, specific airway conductance (s G aw) and multiple-breath nitrogen washout were measured before and after smoking in six of the subjects. 3. One group of smokers ( n = 4) had a greatly increased mean (± sd ) baseline V T /T I (390 ± 39 ml/s) compared with normal non-smokers and another group ( n = 6) a near normal V T /T I (246 ± 36 ml/s). The first group (‘deep inhalers’) had a significantly higher mean inhalation fraction (volume of inhaled smoke and air/vital capacity; 0·25 ± 0·05). The second group had a lower mean inhalation fraction of 0·14 ± 003 (‘moderate inhalers’). 4. V T /T I decreased to 323 ± 33 ml/s in the post-smoking period in the deep inhalers, whereas it increased to 345 ± 65 ml/s in the moderate inhalers during smoking. No systematic difference in V T /T I was noted between smoking high- and low-tar cigarettes, although the high-tar brand tended to have a greater effect on V T /T I . Deep inhalers showed a fall in V T during and after smoking. Moderate inhalers showed a decrease in T I /T TOT during and after smoking. 5. s G aw, measured in four moderate inhalers and two deep inhalers, fell in all subjects after smoking HTC (19 ± 8·7%; P < 0·01) and LTC (13·9 ± 10%) cigarettes. 6. ‘Sham’ smoking with an unlit cigarette produced no change in breathing pattern. 7. In moderate inhalers, the increase in respiratory output (as reflected by increased V T /T I ) combined with a reduction in T I /T TOT appears to reflect the respiratory-centre stimulant effect of nicotine directly or indirectly through broncho-constriction. This contrasts with the reduced neural drive in deep inhalers, which may relate to some overriding satiating effect of the smoking.