Eight asthmatic out-patients with a history of exercise-induced asthma (EIA) were randomly treated with intravenously administered enprofylline 1.5 mg/kg b.wt., theophylline 5 mg/kg b.wt., and placebo immediately prior to a 6-min exercise provocation in this double-blind crossover comparison. A reduction in peak flow of more than 20% was seen in all patients after placebo pre-treatment. Mean plasma concentrations at the start of the exercise test were 3.3 mg/l and 13.2 mg/l after 20 min infusion of enprofylline and theophylline, respectively. The corresponding figures 25 min later were 2.3 and 11.7, respectively. Maximal fall in peak expiratory flow (PEF) after exercise in percent of pre-exercise PEF was 49% +/- 6% (mean +/- SEM), 39% +/- 6% and 24% +/- 5% after infusion of placebo, enprofylline, and theophylline, respectively. Theophylline produced a statistically significant better protection against EIA compared to enprofylline and placebo. Enprofylline produced a slight protection from EIA not statistically significantly different from placebo.