The in vivo concentrations of adenosine triphosphate (ATP), phosphocreatine (PC), and lactic acid are difficult to assess in skeletal muscle. Even when animals are anesthetized, intubated with endotracheal tubes, and given positive pressure respiration during and after administration of d-tubocurarine chloride, subsequent biopsy samples are especially variable (1) in concentration of PC and lactic acid. A considerable decrease in PC and increase in lactic acid may occur during the excision of a sample and prior to cessation of its metabolic activity by freezing in liquid nitrogen or isopentane. The use of a scalpel causes twitching in the muscle, which is accompanied by some breakdown of ATP (2, 3) and a concomitant utilization of PC in the resynthesis of the ATP (4). Additionally the amount of time required to excise a muscle sample and terminate its metabolic activity, is somewhat variable. The present cryobiopsy technique makes it possible to (a) eliminate trauma, and resultant twitching and bleeding, (b) terminate metabolic activity of the tissue before taking the tissue from its blood supply, (c) provide a relatively large sample size, and (d) conduct the operation by personnel untrained in surgical procedures. It is the purpose of this manuscript to describe the cryobiopsy technique as well as to compare PC, ATP, and lactic acid levels in skeletal muscle samples taken by cryobiopsy with those taken through normal biopsy procedures. Description of the cryo-probe. The Freon cooled cryo-probe herein described is a modification of the original probe developed for other applications (5). The purpose of the original development was to provide the surgeon with a less costly, inherently safer cryoprobe than the liquid nitrogen units now available. In this cryo-probe the liquid Freon (Freon-22) is moved by gravity from the supply bottle to the probe (Fig. 1). The probe contains an on-off valve which allows the liquid Freon to flow through a centrally located hypodermic tubing (0.026 in. i.d.) to the probe tip.