Aminophylline in a low dose has been shown to reverse diazepam sedation. The present investigation was performed as a double-blind study to compare the effects of aminophylline and enprofylline on deep diazepam sedation after surgery. Enprofylline is a xanthine derivative with anti-asthmatic effect but, in contrast to aminophylline, enprofylline has very weak adenosine antagonistic properties. A comparison should make it possible to evaluate if adenosine is involved in the observed effects. Twenty male patients undergoing transurethral surgery in spinal anaesthesia were given diazepam during surgery to maintain a state of deep sedation. Postoperatively aminophylline or enprofylline was given from coded ampoules in equipotent anti-asthmatic doses (4.5 or 1.5 mg/kg). The degree of sedation was assessed prior to and after the injections. A difference between the groups was obvious. Patients given aminophylline showed rapid reversal of sedation, persisting throughout the 2-h observation period. Following enprofylline, a markedly slower reversal of sedation was observed. It is likely that aminophylline antagonises diazepam sedation by blocking adenosine receptors. Some clinical implications are outlined.