The pattern of respiration in infants during anaesthesia is not well documented. In this study, minute ventilation (MV) during elective mask halothane anaesthesia (HA) was measured during spontaneous ventilation in infants (Group I) and children (Group II). Airflow was measured with pneumotachography (#0 Fleisch in Group I and #1 Fleisch in Group II). Analogue signals of pressure and flow were recorded on magnetic tape for off-line playback. The flow signal was mathematically integrated to volume. The surgical procedure was divided into three stages: A, B and C representing HA, surgical stimulation and emergence respectively. The pattern of respiration during spontaneous ventilation was described as tidal volume (VTx), respiratory frequency (fx), mean inspiratory flow (VT/TIx), inspiratory duty cycle (TI/TTotx) where the subscript x denoted the stage. Seven infants (2.7 +/- 0.5 mo, 5.8 +/- 0.5 kg) and five children (3.1 +/- 1.1 yr, 15.8 +/- 1.7 kg) were studied. There was no difference in MV between Groups I and II. Halothane anaesthesia in both groups was characterized by rapid shallow breathing: VTA was lower in Group I (2.90 +/- 0.8 ml.kg-1) than in Group II (3.74 +/- 0.40 ml.kg-1) (P < 0.05). Tidal volume was lower during anaesthesia than emergence in both groups (P < 0.05). There was no difference in VT/TIx between groups. The VT/TIA was lower than VT/TIC in Group I (P < 0.05) but not in Group II. There was no intra or intergroup difference in TI/TTot between stages. We suggest that during HA infants have a greater reduction in VT than children, which may predispose infants to hypercarbia during HA.