The hysteretic and reversible polarity-dependent resistive switch driven by electric pulses is studied in both Ag/Pr0.7Ca0.3MnO3/YBa2Cu3O7 sandwiches and single-layer Pr0.7Ca0.3MnO3 strips. The data demonstrate that the switch takes place at the Ag-Pr0.7Ca0.3MnO3 interface. A model, which describes the data well, is proposed. We further suggest that electrochemical diffusion is the cause for the switch.Pr 0.7 Ca 0.3 MnO 3 (PCMO) has attracted extensive interest recently. Below 150 K, its free energies corresponding to the paramagnetic, the charge-ordered, and the ferromagnetic states differ only slightly. Therefore, a slight external disturbance, e.g. magnetic field, light, isotope mass, pressure, or electric field, may lead to a large resistivity (ρ) change, but only at low temperatures.1 Therefore, it is interesting to note the report of Liu et al.2 that the two-lead resistance, R, of a PCMO layer sandwiched between an Ag top-electrode and a YBa 2 Cu 3 O 7 (YBCO) or a Pt bottom-electrode can be drastically and repeatably alternated at room temperature by applying electric pulses with different polarities.3 This R-switch may thus offer potential device applications, e.g. nonvolatile memory. Similar R-changes in single-layer PCMO films with the four-lead configuration were also reported. The R-switch has therefore been attributed to bulk properties of PCMO, in terms of the alignment of the presumed ferromagnetic clusters by the electric field.2 The interpretation, if confirmed, presents a major challenge to the physics of manganites and, possibly, to the basic law of parity conservation. The reported R-change of ∆R ≥ 3000 Ω across a 600 nm thick PCMO film represents a ρ-increase of ∆ρ ≈ 105 Ω cm, and suggests a novel state with a ρ(297 K) far greater than the ρ(297 K) << 10 1 Ω cm ever reported in PCMO. According to the commonly accepted polaron model, ρ(297 K) of manganites is controlled by the polaron mobility and should be ultimately restricted by the hopping barrier (10 −1 eV ≈ k B T at 297 K) associated with the Jahn-Teller distortion, which is only a few eV. 4 The experimental ρ(297 K) is only 10 −2 to 10 0 Ω cm in (La y Pr 1−y ) 1−x Ca x MnO 3 for 0.2 ≤ x ≤ 0.5 and 0 ≤ y ≤ 0.7, 5 and ≤ 10 4 Ω cm even in extreme cases, such as Nd 0.7 Ba 0.3 MnO 3 and LaMnO 3 .6 A ρ(297 K) of 10 5 Ω cm or higher would suggest a new insulating state never observed before and challenge the polaron model commonly accepted. In a more general sense, this polarity-dependent ρ in a uniform material reported, if proven, represents a violation of the law of parity conservation in the electromagnetic field. It may occur without parity violation only if the sample is asymmetric due to either an inhomogeneity in the thickness direction or poling by electric pulses ("training"); neither bears any obvious relation to the alignment model proposed.2 The present study is motivated by our attempt to elucidate the mechanism responsible for, and the nature of, the R-switch. Our data demonstrate that the switch occurs at the Ag-PCMO interface, ...