The polymerization of 4-vinylpyridine (4VP) in an electrochemical cell in liquid NH3 solutions was studied. With monomer concentrations of 5 to 25 vol.-% in the temperature range −33 to −78°C. rapid polymerization takes place when current is passed through the cell. Polymer yields of 80 to 90% are easily obtained in a few hours with NaN3 or NaCl as electrolyte and smooth Pt electrodes in a stirred three-compartment cell. Current densities of 1 × 10−2 to 10 ma./cm.2 and applied voltages of 2.5 to 50 v. were employed. Different anodic materials were used, including Pt and pyrolitic graphite. The electrochemical yield is about 1 mole of polymer per faraday for current densities of ⩽ 1 × 10−2 ma./cm.2, and it becomes 0.5 and lower at higher current densities. The rate of polymerization was found to increase, and the molecular weight of the polymer to decrease, with increasing current density. It is concluded that the initiation step consists of electron transfer between the cathode and the monomer. Depending on the amount of polymer deposited on the cathode and on the current density, initiation may occur on the cathode surface or within the polymer deposit on the cathode or in the free liquid solvent. The consequence of the electron transfer to monomer is the formation of radical anions. Electrochemical yields of about 1.0 mole of polymer per faraday suggest that radical anion dimerization does not take place extensively, while dimerization may predominate when electrochemical yields are about 0.5. Subsequent chain growth occurs through an anionic mechanism, as evidenced by the nonterminating nature of the polymerization, the presistent red color of the polymer formed, and the sensitivity of the polymerization to the addition of protic substances. Polymer molecular weights were in the range of 10,000 to 360,000. Molecular weight distributions with Mw/Mn ratios of 1.21 or larger were found. Softening points, solubility characteristics, and infrared spectra of the electrochemical polymer are presented and discussed in comparison with those of poly(4-vinylpyridine) prepared with chemical initiators.