The production of constrictive responses limited to either arterial or venous segments of the vasculature of the dog paw was tested by stimulating various peripheral nerves of the hind leg. Resistance changes of the total vasculature of the paw, and of three of its component series-coupled segments (arterial, small vessel, and venous) were studied sequentially before and during stimulation of the sciatic nerve and of three of its branches (tibial, deep, and superficial fibular nerves). Stimulation characteristics (15 to 90 volts, 1-3 msec duration, 25/sec, and 30-second stimulation periods) were chosen to produce maximum vasoconstrictive responses. Tibial and deep fibular nerve stimulations produced constriction of the arterial segment but not of the venous segment. Superficial fibular nerve stimulation produced constriction of the venous segment but not of the arterial segment. All nerve stimulations produced constriction of small blood vessel segments. Thus, responses could be limited to either arterial (plus small vessel) or venous (plus small vessel) segments. Inability to localize response within small-vessel segments made it impossible to determine whether any peripheral nerve produced constrictive responses limited strictly to either pre- or postcapillary segments. No relationship was evident between control resistance values and the subsequent changes in resistance occurring in response to the standard stimulations.