Patients with Von Hippel-Lindau disease were investigated and the following were shown: investigation of the molecular biology of hemangioblastomas of the central nervous system (CNS) revealed an homozygous deletion of a portion of the short arm of the third chromosome, demonstrating that these tumors are probably caused by the absence of a tumor-suppressing gene, as are familial retinoblastomas. MRI with gadolinium-EDTA contrast enhancement was shown to be a sensitive technique for detection of small hemangioblastomas of the CNS. Excision of the tumors alone was shown to result in resolution of syringomyelia associated with spinal cord hemangioblastomas. Therefore, the tumor- associated syrinx does not need separate treatment. Endothelial-derived relaxation factor (nitric-oxide) was shown to mediate autoregulation and chemoregulation of cerebral blood flow. Nitricoxide synthase immunoreactivity was demonstrated in the nerve plexus in the adventitia of the Circle of Willis in primates. During cerebral vasospasm in primates, disappearance of immunostaining for nitrioxide synthase from the adventitial plexus of the involved artery was demonstrated. Immunoreactivity returned after resolution of cerebral vasospasm Cerebral blood flow in the distribution of cerebral vasospasm could be increased by intracarotid infusion of L-arginine, the substrate for nitric oxide production. These findings suggest that regulation of nitric oxide production is impaired in cerebral vasospasm and that this impairment may underlie the etiology of cerebral vasospasm.