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The aim of this study was to investigate the possibility of detecting root canal constrictions by using an apex locator. Seventy impenetrable canals in extracted human teeth were used. Based on contact microradiographic findings, they were divided into three groups: group A (constricted within 3 mm of the radiographic apex, 23 canals), group B (nonconstricted within 3 mm of the radiographic apex, 28 canals), and group C (constricted more than 3-mm short of the radiographic apex, 19 canals). Electrical impedances at two different frequencies (8 kHz and 0.4 kHz), as well as the meter values of the Root ZX were recorded at the position where a file tip could reach. The impedance ratios (Z8kHz:Z0.4kHz) and the meter values of the Root ZX showed a statistically significant difference between groups A and B. It was suggested that the Root ZX might be useful for detecting root canal constrictions.