A study was made of the effects of inhaling 5 mg of terbutaline and 0.5 mg of ipratropium bromide in 11 patients arriving at our emergency room with acute asthma (FEV1 less than or equal to 50% of the predicted). Measurable plasma levels of terbutaline before treatment were found in all patients who reported having taken oral terbutaline (mean value 30 nmol/l, range 11-89). A significant correlation was found between the reported terbutaline medication and the measured terbutaline plasma concentration (p less than 0.01). Plasma terbutaline had increased by 6-20 (mean 15) nmol/l 60 min after the start of treatment and by 6-45 (mean 14) nmol/l at 120 min, compared with the pre-treatment value. A highly significant decrease in dyspnoea and an increase in PEF and FEV1 was measured (p less than 0.01) after treatment, while no significant changes in respiratory rate, pulse rate, blood pressure or tremor were recorded. A significant positive correlation was found between delta plasma terbutaline and delta systolic blood pressure 120 min after treatment (p less than 0.05), but apart from this no statistically significant correlations were found between plasma terbutaline on arrival or delta plasma terbutaline and the other measurements of the effect of treatment. One of the advantages of adding ipratropium to nebulised beta-agonist treatment might be that it permits the use of lower doses of beta 2-agonist and thereby reduces the systemic side-effects of the treatment.