Nine preterm infants with hyaline membrane disease were studied using a ventilator triggered from abdominal movement. It was possible to alter respiratory rate over a short space of time by adjustments of the inspiratory time setting. There was a marked inverse relationship between inspiratory time and both ventilator and baby's respiratory rate--mean baby respiratory rate was 62 breaths/min at an inspiratory time of 0.2 s and 45 breaths/min at 0.8 s. This drop was statistically significant (P less than 0.005). Mean tidal volume changed little over this range. This interaction meant that mean minute ventilation was optimal at inspiratory times of 0.2 to 0.4 s, being 269 and 258 ml/kg per minute, respectively, but at 0.8 s fell to 213 ml/kg per minute (P less than 0.05).