The mineral insulators, such as Al2O3, MgO, and SiO2, appear to possess satisfactory radiation tolerance and acceptable high temperature electrical properties for many nuclear applications. However, our tests on practical cables for reactor instrumentation systems, and experiments to determine electrical properties of bulk materials, have revealed phenomena which limit applicable temperature ranges and dc voltage bias levels. For example, these materials exhibit a rapid decrease in electrical resistivity and a corresponding increase in law frequency dielectric constant with increasing temperature above some critical temperature. Also, cables employed for dc bias purposes have exhibited a phenomenon, termed breakdown pulse noise, which has been a major obstacle to satisfactory operation of low-level fission counters for the Fast Flux Test Facility since the breakdown-pulse -noise -generated pulses are indistinguishable from typical detector pulses. SiO2- and MgO-insulated cables determined a useful region of operation as a function of insulator compaction density and purity. However, if insufficient care is taken in the preparation and design of cable end seals, the connector region of the cable may cause unsatisfactory performance even though the body of the cable satisfies test criteria.