Percutaneous transluminal angioplasty for treatment of cerebral vasospasm was performed in primates. Chronic cerebral vasospasm was induced by placement of an autologous blood clot over the right internal carotid artery (ICA), middle cerebral artery (MCA), and anterior cerebral artery (ACA). Cerebral angiography on Day 7 showed that the diameters of the ICA, MCA, and ACA were reduced to 55.7% +/- 1.3%, 55.3% +/- 2.6%, and 59.6% +/- 1.3%, respectively, of baseline. The angioplasty was carried out with a silicone microballoon attached to a microcatheter under somatosensory evoked potential (SEP) monitoring on Day 7. The angioplasty for ICA was performed satisfactorily; however, the balloon could not be advanced to the spastic M1 or A1 portions of the cerebral artery. Following angioplasty, the diameters of the ICA, the M1 segment, and the A1 segment were 79.6% +/- 2.9% (p < 0.001), 67.6% +/- 4.3% (p < 0.05), and 61.7% +/- 2.2% (not significant), respectively, of baseline. Histological studies demonstrated that the vessels were well dilated and patent without endothelial cell damage.