Respiratory resistance significantly increased from 5.0 to 8.0 cm H2O/1/sec in anesthetized patients who were given ultrasonically nebulized water for 20 minutes via an endotracheal tube. Intravenous administration of lidocaine failed to reverse the provoked increase in resistance. In another group, respiratory resistance significantly increased from 5.8 to 7.5 cm H2O/1/sec in response to nebulized water despite prior and con-current intravenous administration of lidocaine. In a third group, initial respiratory resistance was 5.6 cm H2O/1/sec and did not increase during a 20-minute challenge with intratracheally administrered ultrasonically nebulized 2 per cent lidocaine. In a final group, resistance was increased from 5.0 to 6.9 cm H2O/1/sec with nebulized water. When challenge was continued with nebulized 2 per cent lidocaine, resistance remained elevated for about 10-12 minutes. It then decreased and returned to its initial control value at about 17 minutes, despite continuing lidocaine aerosol administration. Lidocaine, when administered intratracheally as an aerosol, both prevented and reversed provoked increases in respiratory resistance. Intravenously administered lidocaine was ineffective. Intratracheal administration of ultrasonically nebulized lidocaine might be another useful technique for management of bronchoconstriction in anesthetized patients.