Study Design. This study determined the predictive ability of electrical impedance measurement in detecting cortical perforation in a porcine model of pedicular exploration. Objective. This study tested the hypothesis that a large decrease in electrical impedance would occur as a result of perforation of the vertebral cortex by the pedicle probe. Summary of Background Data. The resistivity of cortical bone has been reported to be 25 to 100 times greater than that of soft tissues. Methods. A total of 42 pedicles of the lumbar spines of six swine were explored using the instrumented pedicle probes. Results. Using a 1-mu Amp 30-Hz current source, measurement of electrical impedance predicted cortical rupture with a sensitivity, specificity, and accuracy of 95\%. Maximum applied voltages of 2.8 mV did not result in myogenic stimulus. Conclusions. Electrical impedance measurement provides an accurate real-time measurement of cortical perforation. This technique is adapted readily for use with pedicular screws and screw taps. Further investigation to determine the clinical use of this technique is recommended.