This article presents the results of conductivity and density measurements of liquid, gaseous, and supercritical mercury up to 1700°C and 2100 bar. Beyond the critical temperature of 1490°C the conductivity varies continuously from 10-4 to 102 Ω-1 cm-1 if the density is increased from 2 to 6 g/cm3. This variation is an example of nonmetal-metal transition and is discussed accordingly. The activation energy of conductivity in the supercritical fluid decreases markedly with increasing density and approaches very small values around 5 g/cm3. Conductivity data for dense gaseous mercurycesium mixtures are given.