Electrically conductive acrylonitrile butadiene rubber compounds filled with different concentrations of fast extrusion furnace black were experimentally investigated. The percolation concentration of the investigated composites was found to be 65 phr. Samples below percolation concentration are PTCC and above percolation concentration are NTCC. Sample N70 that belongs to the percolation concentration exhibits NTCC and PTCC. The effect of heating-cooling cycles shows that samples with high concentration have small hysteresis compared with low concentrations. (c) 2006 Wiley Periodicals, Inc.