This invention relates to a medical clamp for controllably occluding extremely small body vessels such as blood vessels within the intracranial or intraspinal areas of the human body. The disclosed clamp is preferably formed from a U-shaped unitary body of transparent, resilient elastomeric biologically inert material such as Tygon or Silastic wherein the leg members of the U-shaped body are controllably brought together to occlude a vessel placed therebetween by means of a flexible line or spring. Several embodiments are disclosed illustrating various expedients for securing the flexible line such as a sliding friction device or latch plate, and for operating and securing the string mechanism. All embodiments include means for measuring and detecting the termination of fluid flow through a vessel being occluded. Such measuring or detecting means permit the use of only that amount of pressure and no more than is necessary to cause vessel occlusion.