Seven SUNCT patients (six men, one woman) took part in this study. In four patients, respiratory variables were compared during and outside attacks. In five patients, peripheral chemosensitivity was tested and compared with a control group matched with respect to age, sex, and smoking habits. The results indicate that SUNCT patients hyperventilate during attacks. Moreover, they appear to hyperventilate slightly under basal conditions. The tests for peripheral chemoreceptor activity indicated no differences between the SUNCT and the control groups except for one variable, namely the mean ventilatory response to a single breath of 13% CO2. It is possible that this indicates a blunted response of the peripheral chemoreceptors. On the other hand, it may also represent a chance finding, since none of the other results presented suggested such a conclusion, and the size of the test group was very small. The results do not indicate that a reduction in oxygen saturation can trigger SUNCT since low levels of oxygen saturation were only rarely accompanied by SUNCT, whereas many attacks were not associated with any appreciable lowering in arterial oxygen saturation.