Both ac and dc conductivities of nominally pure nanocrystalline ZnO ceramics with grain size of ~ 40 nm were measured as a function of oxygen partial pressure and temperature, and compared with coarsened microcrystalline samples (grain size of ~ 5 μm). Nanocrystalline ceramics showed intrinsic bulk properties which turned into extrinsic behavior on coarsening. This is attributed to extreme impurity segregation into grain boundary core in nanocrystalline ceramics owing to the markedly enhanced interface-to-volume ratio. Nonetheless the blocking effect (Schottky barriers) of the grain boundaries in the nanocrystalline sample was negligible compared to that in the coarsened material due to the extreme dilution of the interfacial traps. Investigations on dopant behavior in ZnO nanowire bridges based on high temperature electrical measurements are currently under progress.