The effect of nebulised iso-osmolar, preservative free, but acidic salbutamol solution was studied in 34 acutely wheezing infants aged 1-17 months. Transcutaneous oxygen pressure (TcPO2) and oxygen saturation (SO2) fell significantly during the first five minutes after nebulisation with further deterioration at 15-20 minutes. Ten of these infants were followed up for another two hours and showed slight improvement. Even after the second hour TcPO2 had not reached baseline values. Three months later the response to salbutamol and a placebo of equal acidity (pH 3.9) was studied in 11 infants from the same group, now free of symptoms. Lung function tests were included and showed no significant changes in specific conductance and volume corrected maximum expiratory flows (Vmax at functional residual capacity/thoracic gas volume). However, hypoxaemia occurred after the acidic placebo with a significant drop of TcPO2 (mean 0.9 kPa); SO2 decreased similarly but this did not reach significance. After salbutamol there was a further significant deterioration of mean TcPO2 (1.4 kPa) and of SO2. These results show that beside a possible pharmacological effect of salbutamol the acidity of the aerosol also induces hypoxaemia in infants.