The effects of pancuronium bromide on the cardiovascular system and fluid balance in preterm ventilated neonates was investigated. Infants were selectively paralysed if actively expiring. Their results were compared to a control group of similar gestational age, ventilated during the same study period, but who received no paralysing agent. There was no significant difference in the blood pressure of the two groups. Treatment with pancuronium was not associated with differences in blood pressure variability. All paralysed infants showed a significant reduction in heart rate variability which persisted during treatment. This was not present in the control infants. Paralysed infants, despite fluid restriction, had evidence of fluid retention, being significantly heavier than the control babies from day 3 onwards and above their birthweight by day 7. No paralysed or control infant developed a pneumothorax or renal failure. We conclude that selective treatment with pancuronium is without adverse effects on blood pressure but strict attention to fluid balance is essential.