Complex impedance analysis at cryogenic temperatures has revealed that the bulk and grain boundary properties of BaTiO3 polycrystals are very sensitive to the oxygen partial pressure during sintering. Polycrystals sintered at P-O2 as low as 10(-15) atm were already electrically heterogeneous. The activation energy of the bulk conductivity in the rhombohedral phase was found to be close to that of the reduced undoped single crystal (i.e., 0.093 eV). The activation energy of the grain boundary conductivity increases with the temperature of the postsinter oxidation treatment from 0.064 to 0.113 eV. Analysis of polycrystalline BaTiO3 sintered in reducing atmosphere and then annealed at P-O2 = 0.2 atm has shown that the onset of the PTCR effect occurs at much higher temperatures than expected in the framework of the oxygen chemisorption model. The EPR intensity of barium and titanium vacancies increases after oxidation at T > 1000degreesC. A substantial PTCR effect is achieved only after prolonged annealing of the ceramic in air at temperatures as high as 1200-1250degreesC. This result suggests that the PTCR effect in polycrystalline BaTiO3 is associated with interfacial segregation of cation vacancies during oxidation of the grain boundaries.