Abstract: A cryosurgical instrument having a cooling mode and a rapid warming or defrost mode is provided with a first conduit through which refrigerant gas is metered to undergo a Joule-Thomson or isoenthalpic expansion adjacent the tip of the instrument to cool the tip. The tip is defrosted by admitting a larger volume of unexpanded gas from the source to the tip through a second conduit and back pressurizing the gas to provide a sufficient balance to cause it to undergo heat exchange with the tip. Valve means, upstream from the tip is used to selectively control the gas flow path through either the first or second conduit. The tip is in communication with an exhaust port constantly open to the atmosphere. A pulse valve sensitive to gas volume in the exhaust line is positioned downstream from the tip between the tip and exhaust port to aid in back pressurizing the unexpanded gas by partially blocking the exhaust line.