It has been reported that the quantity of fibrillation potentials (FP) decreases with drop in the intramuscular temperature. However, the quantitative measurement of the FP with intramuscular temperature changes has not been reported. Under anesthesia of intraperitoneal sodium pentobarbital, the sciatic nerve of 6 rats (Sprague-Dawley) was surgically isolated. A 1-cm segment was excised after tying the proximal and distal ends of the nerve segment. A concentric needle electrode and thermometer needle probe were inserted approximately 1-cm apart into the posterior tibial muscles 3 to 4 days after the nerve segment excision. Before and during cooling the muscles with ice, FPs were evoked and printed on paper recording for later analysis. Visually recognizable potentials (30 microV or above) in each printed tracing with temperature changes (range, 37 degrees C to 15 degrees C) were manually counted. A positive linear correlation was found between the intramuscular temperature changes and the quantity of FPs. The recording of FP completely ceased at about 20 degrees C below baseline temperature. In this study, we have successfully semiquantified fibrillation potentials in a wide range of intramuscular temperature changes.