Measurements have been made of the impedance/frequency characteristics of a hot‐cathode, dc, mercury‐vapor plasma at a pressure of approximately 1 &mgr;. The experimental tube contained a directly‐heated cathode from a commercial mercury‐vapor rectifier and provided a discharge column 80 cm long. Bridge techniques were used to obtain impedance loci extending to 2 Mc/sec for several values of anode current. These appeared to normalize, the magnitudes of the components being approximately inversely proportional to current. A basic equivalent circuit can be deduced, its parameters being dependent on physical processes occurring in the discharge. Prominent frequency components of the low‐frequency voltage fluctuations present in the tube appear to be closely related to the impedance characteristics.