Physiologic mechanisms involving local vasoregulating factor, endothelium-derived relaxing factor (EDRF) and endothelium-derived constricting factors (EDCF) have been postulated to play a role in VWF. Recent evidence that endothelin is a potent vasoconstrictor peptide indicates that it may play a role in vasoregulation during vibration exposure through the local actions of EDRF or EDCF. Therefore, we examined the effects of grasping (50 N) and hand-arm vibration with an unweighted acceleration of 50 m/s2 rms at a frequency of 120 Hz in the direction of X-axis on digital blood flow (DBF) and on the level of plasma endothelin in 7 healthy male office workers. DBF was measured by an apparatus based on the thermal diffusion method where the thermal diffusion flow probe was incorporated in the Peltier's stack. In the grasping test, DBF decreased by about 40% from resting level within 1 minute after the grasping was begun, and was maintained at almost the same level during grasping, but increased immediately after cessation of grasping. In the vibration exposure test, initial decrease in DBF due to grasping was the same as that observed in the grasping test, but DBF gradually increased when exposure to vibration was performed simultaneously. No significant change in plasma endothelin level was induced by grasping. Endothelin was significantly lower after exposure to vibration than at rest and after grasping. The grasping-induced decrease in DBF seemed to be due to mechanical compression of the vessels. The negative correlation between DBF and endothelin during vibration exposure suggests that a reduction in release of endothelin from smooth muscle into the vessel cavity during vibration leads to vasodilatation, possibly attributable to the local axon reflex.