Plasma lidocaine concentrations were intermittently measured in 8 upper gastrointestinal endoscopy and 12 bronchoscopy patients. The highest individual concentration was 0.98 microgram/ml in the upper gastrointestinal endoscopy patients and 3.79 microgram/ml in the bronchoscopy patients. Highest concentrations were reached at 15 minutes in the gastrointestinal endoscopy patients and at 30 or 60 minutes in the bronchoscopy patients. Thus, since lidocaine does not produce toxic effects at concentrations inferior to 6 microgram/ml, doses of this topical anaesthetic up to 16 mg/kg can be safely given during endoscopic procedures to patients with normal hepatic and cardiovascular functions. However, patients with liver metastases should be considered at high risk even if their liver function tests are normal. Patients at high risk of developing lidocaine toxicity should receive lower doses and be closely watched for at least 60 minutes after the end of the procedure.