A positive temperature coefficient of electrical resistivity (PTCR) in ZnO-NiO ceramics was observed and an attempt to explain its origin was made. According to electrical measurements, supported by dilatometric and microstructural investigations, the percolation principle was used for this purpose. A series of samples with different ZnOss and NiOss contents were prepared. The electrical resistivity of the samples was measured at different temperatures. ZnOss represents a low ohmic and NiOss represents a high ohmic phase. Monophase samples exhibited a strictly negative temperature coefficient of electrical resistivity (NTCR), while in some two-phase samples PTCR behaviour was observed. Results of electrical, dilatometric measurements and microstructural investigations indicated a possible explanation of PTCR behaviour on the basis of the percolation model.