The response and plasma levels of inhaled (0.15 mg.kg-1) and intravenous salbutamol (5 micrograms.kg-1) were studied in 12 patients who had previously been included in a study of acute severe asthma and were now in a stable phase. The maximal value for change (delta) in plasma salbutamol compared with the pretreatment value was higher after i.v. than after inhalation treatment (59 vs 30 nmol.l-1) but the areas under the curve (AUC) during 90 mins after treatment were similar for the two administration routes. The inter-individual differences in peak plasma salbutamol and AUC for delta plasma salbutamol were much greater after inhalation than after i.v. infusion. Immediately after treatment there was a greater increase in pulse rate (mean +17 vs +6 beats.min) and lower serum potassium level (mean -0.3 mmol.l-1) after i.v. than after inhalation treatment. The increase in peak expiratory flow (PEF) measured 90 min after the start of treatment (85 vs 31 l.min-1) and as AUC was significantly higher after inhalation than after i.v. treatment. Six patients had received i.v. treatment in the acute asthma study: in these patients delta PEF immediately after i.v. treatment was significantly lower in the stable situation (+49 vs +115 l.min-1). There was also a significant difference in the effect on the pulse rate immediately after i.v. treatment, with a decrease in the acute asthma and an increase in the stable asthma situation (-5 vs +16 beats.min-1).