The dielectric constantse′ and loss factors,e″, for pure single-crystal MgO and for Fe-and Cr-doped crystals have been measured at frequencies, ω, from 500 Hz to 500 kHz at room temperature. For pure MgO at 1 kHz the values ofe′ and the loss tangent, tan δ, (9.62 and 2.16×10−3, respectively) agree well with the data of Von Hippel; the conductivity, σ, varies as σ ∞ ω n withn=0.98±0.02. In Fe-doped crystals σ increases with Fe-concentration (at any given frequency); for a crystal doped with 12800 ppm Fe, σ was about four times the value for pure MgO. At all concentrations the variation of log σ with log ω was linear andn=0.98±0.02. A decrease ine′ with increasing Fe-concentration was also observed. A similar, although less pronounced, behaviour was found in Cr-doped crystals. The effects are discussed in terms of hopping mechanisms.