A listing is given of the characteristics of high-temperature ceramics used widely in elemental vapor lasers. It is shown that a ceramic based on pyrolytic boron nitride has the best characteristics as judged by a number of criteria. The apparatus for measuring low- and high-voltage electrophysical characteristics of insulators in various gases kept at pressures of 1-10**5 Pa at temperatures up to 2500 degree K is described. The experimental values of high-voltage electrophysical characteristics of boron pyronitride are reported for the first time.