SUMMARY. Spinal manipulative therapy techniques are frequently applied by physiotherapists to relieve pain of musculo-skeletal origin and to improve the quality of joint movement in a variety of musculo-skeletal conditions. However, there has been little research into the physiological effects of these techniques, or the mechanisms responsible for these effects. The aim of this study was to establish whether a grade III posteroanterior mobilization technique applied centrally to the cervical spine would affect respiratory and cardiovascular indicators of sympathetic nervous system function in pain-free, normal volunteers. A significant increase in respiratory rate, heart rate, systolic and diastolic blood pressure occurred during application of the technique to C5/6, when compared to the control and placebo conditions. There was little change in any of the measured variables during the placebo condition. This study provides objective evidence that application of this mobilization technique elicits changes in sympathetic nervous system activity distinct from placebo in pain-free individuals. These results provide a basis for further research into the physiological effects of manipulative procedures, and in particular, exploration of the mechanisms responsible for analgesia produced by this method. Copyright 1997 Harcourt Publishers Ltd.