1. The renovascular effects of neuropeptide-Y (NPY) were examined in the split hydronephrotic rat kidney. 2. Systemic infusion of low non-pressor doses of NPY (0.2 micrograms kg-1 up to 5.0 micrograms kg-1) produced a non-uniform pattern of vascular reactivity. In general, a significant constriction of the proximal and distal arcuate artery was seen at all doses. No constriction was seen at the interlobular artery or the larger part of the afferent arteriole. These segments initially dilated during the lower dose infusions. The very distal part of the afferent arteriole adjacent to the glomerulus and the proximal efferent arteriole responded in a similar way to the arcuate arteries. 3. NPY, locally applied into the tissue bath at concentrations of 1 nmol l-1 up to 25 nmol l-1, produced non-uniform vascular reactions similar to those of intravenously infused NPY. At the considerably higher local dosage of 1.14 mumol l-1, all vascular segments revealed vasoconstriction. 4. NPY application did not attenuate effects of acetylcholine. This observation suggests that the mechanism of NPY-induced vasoconstriction does not rely upon antagonism of endothelium-derived vasodilatation. 5. The pattern of vascular reactivity to NPY was substantially different from that known for the vasoconstrictors noradrenaline and angiotensin II in our preparation.