Digital artery blood flows were compared with digital pad opacity pulse amplitudes in dogs during vasoconstrictor responses. A high correlation between digital artery flow and opacity pulse amplitudes occurred during infusions of epinephrine and sciatic and tibial nerve stimulations, but not during superficial fibular nerve stimulation. The results confirmed earlier findings that tibial nerve stimulation produced predominantly precapillary constriction, and that superficial fibular nerve stimulation produced predominantly postcapillary constriction. Events in precapillary vessels appeared to be responsible for the opacity pulse. This fact probably forms the basis for the close relationship between opacity pulse amplitude and blood flow.