Transmission electron microscopy has been used to study defects in as‐grown MgO single crystals and in crystals which had undergone high‐temperature electrical conduction for long periods of time. Besides dislocation loops of (a/2⟨110⟩) Burgers vectors and precipitates, a new type of defect, dislocation loops of &agr;⟨100⟩ Burgers vectors lying in {100} planes, has been observed in electric‐field‐treated specimens. The nature of both (a/2) ⟨110⟩ anda⟨100⟩ loops was determined to be vacancy type. The relation of this microstructure to the observed increase in electrical conductivity, which ultimately results in thermal‐electric breakdown of the samples, is discussed.