The effects of the withdrawal of nitrous oxide from the inspired gas mixture were studied in 10 spontaneously breathing children during nitrous oxide-halothane anaesthesia, before and during surgery, using a computerized system for the measurement, recording and analysis of data. Before surgery the decline in the alveolar nitrous oxide concentration was associated with an increase in minute ventilation (32.7%, P less than 0.05), and a decrease in alveolar carbon dioxide concentration (8.4%, P less than 0.05). These effects were produced solely by an increase in tidal volume (42.7%, P less than 0.001), as no significant change in respiratory rate was observed. The hypoventilation produced by an alveolar mixture of 60% nitrous oxide and 0.9% halothane, a reduction of VE by 50%, exceeded the hypoventilation caused by 0.9% halothane alone, which reduced VE by 36.6%; and the hypoventilation produced by nitrous oxide and halothane was rapidly reversed by the withdrawal of nitrous oxide from the inspired gas mixture. During surgery all indices of ventilation were stimulated, and there was greater variability of response, but the pattern and degree of change in response to nitrous elimination, VE increased by 33.3%, VT by 33.8%, closely resembled the changes before surgery. Five children had received papaveretum as premedication, and five thiopentone per rectum; the depression of carbon dioxide responsiveness was more severe in the group who received papaveretum, and their responses to nitrous oxide elimination were less than, and occurred later than the responses in the group given thiopentone.