In this work, the inter-relationship between the microstructure, the grain boundary electronic structure, and the high- and low-field electrical properties of the positive temperature coefficient of resistivity BaTiO3 ceramic thermistors was explored. At high temperatures, the resistance of the material is strongly dependent on the field strength. This is an intrinsic feature of the double Schottky barrier structure of the grain boundaries. Low resistivity thermistors (p approximate to 10-100Ohm cm) were prepared by standard ceramic processing routes. Low-field properties show a large resistivity jump starting at temperatures above 130degreesC. The resistivity under high fields (up 1V per grain boundary, V-gb) was measured in the insulating state at elevated temperatures (180degreesC < T < 220degreesC). High-field data displayed good agreement with the model for transport, dominated by thermionic emission. It was also observed that the combination of grain boundary dopants (e.g. Mn) and high doping levels (La concentration greater than or equal to0.35 mol%) significantly improved the high-field resistance at the cost of an increase in the room temperature resistance. In addition to grain size effects, this behaviour was attributed to the presence of deep-level grain boundary states caused by the presence of Mn.