We have measured total work of breathing before and after the inhalation of water, salbutamol, and ipratropium bromide, given as nebulised solutions, in 39 studies on 25 infants with acute, severe bronchiolitis. Twenty minutes after nebulised water, mean work of breathing per minute was increased by 4% and work per litre by 10% with 2 infants having significant improvement and 2 others showing deterioration. After salbutamol, mean work of breathing per minute showed a 22% increase and work per litre a 0.5% rise. The condition of only one child improved by greater than 25% after this drug. Ipratropium bromide led to significant improvement in 6 out of 15 studies and no corresponding deterioration. The group results showed a fall in work of breathing, 18% in work per minute and 16% in work per litre.