The bronchodilating effect and side-effects of theophylline and a beta 2-adrenoceptor stimulating drug (terbutaline) alone and in combination were studied in 10 adult asthmatic patients. Initially, each individual's pharmacokinetic parameters for theophylline were determined. On 3 separate days, theophylline was infused to defined steady-state concentrations (0, 7.5, and 15 micrograms/mL, respectively) followed by the administration, at intervals of 1 h, of incremental intravenous doses of terbutaline. Theophylline caused a plasma concentration-dependent increase in the volume of air expelled in the first second of forced expiration (FEV1). Theophylline itself had no significant effect on objectively recorded skeletal muscle tremor or heart rate but enhanced the terbutaline-induced increase in both. At the highest steady-state concentration (15 micrograms/mL), 2 of the patients experienced nausea. Terbutaline caused a plasma concentration-dependent increase in FEV1, heart rate and tremor. The plasma levels of terbutaline at the given doses did not differ significantly between individuals or within individuals at different theophylline levels. The combination of theophylline and terbutaline resulted in an additive effect on bronchodilatation. At comparable levels of bronchodilatation, the combination of theophylline and terbutaline caused a lesser degree of side-effects than either drug alone.