Adrenaline and noradrenaline was measured just before and just after chest physiotherapy and endotracheal suctioning in 13 preterm, ventilated, newborn infants. Mean aortic blood pressure was also recorded. Eight of the infants received phenobarbitone. Catecholamine levels were five-fold higher in the 5 infants with blood pH less than 7.30 compared to the other 8 infants. After the procedure, both adrenaline and noradrenaline were significantly higher than baseline levels. The adrenaline response to the procedure was a two-fold increase and significantly greater than the noradrenaline response. Analysis of the effects of phenobarbitone treatment and acidosis on catecholamine responses by multiple linear regression demonstrated that the adrenaline response was reduced by phenobarbitone while the noradrenaline response was unaffected. There were no associations of blood pressure, responses with catecholamine responses, with acidosis or with phenobarbitone treatment.