The stimulating and moderating factors of augmented inspirations provoked by artificial ventilation have been studied in 40 preterm babies on 182 different occasions. Provoked augmented inspirations were demonstrated by recording spontaneous respiratory activity during artificial ventilation by means of an oesophageal balloon and a pneumotachograph. Augmented inspirations were only seen during 18% of the study occasions. The frequency of this reflex was inversely related to the lung compliance as was the ventilator pressure necessary to provoke the augmented inspiration. The inflating volume which stimulated the reflex, when related to body weight, was similar in all the babies. Augmented inspirations were always provoked during the rapid rise in ventilator pressure at the onset of each inflation. Neonates who were recovering from paralysis by pancuronium and others being treated with theophylline both showed an increased frequency of augmented inspirations. It is possible that theophylline could be used to harness this advantageous reflex for the benefit of the ventilated preterm neonate.