The effects of upper abdominal surgery on diaphragmatic function were studied in eight supine patients before and after administration of aminophylline. Changes in pleural (ΔPpl) and gastric pressure (ΔPga) swings were measured with balloon catheter systems. Transdiaphragmatic pressure change (ΔPdi) was calculated as the difference ΔPga-ΔPpl. The ratio ΔPga/ΔPdi, used as an index of the diaphragmatic contribution to the quiet breathing process, decreased significantly as early as 1 h after operation without any further change throughout the 6-h period studied. Administration of aminophylline (6 mg/kg), six hours postoperatively, produced a significant increase in this diaphragmatic index. These data indicate that the early reduced diaphragmatic activity, after upper abdominal surgery, partially may be reversed by administration of aminophylline. The mechanism of its action may involve central nervous stimulation and/or a direct inotropic effect on diaphragmatic muscle. Further studies are needed to evaluate if the correction of altered diaphragmatic motion by aminophylline improves postoperative lung function.