In order to investigate the duration of respiratory depression following neurolept anaesthesia, 21 patients who had hemilaminectomia performed were subjected to ventilation-carbon dioxide response tests by a rebreathing method in the post-operative period. Up to 3 1/2 h after start of anaesthesia all patients had a decreased ventilatory response to carbon dioxide. After this period some patients showed a normal respiratory response, while others remained depressed for up to 6 h, the period for which the investigation lasted. Most patients in the depressed group were premedicated with pethidine. We conclude that additional narcotics are contraindicated for a period of 3 1/2 h following the start of neurolept anaesthesia for surgical procedures lasting 1-2 h, while clinical control is still necessary before administration of additional narcotics within 6 h, especially if narcotics have been used as premedication.