Mold-cast, metal-filled composites made of poly(methyl methacrylate) and aluminum or nickel exhibited electrical resistance qualitative behavior as predicted by the percolation and tunneling mechanisms. A precipitous change in resistivity of about 10(4) ohm-cm was found at volume concentrations of filler in the range of 18.5 to 19.8 vol%. Details of the electrode system are given for making these measurements. Using different polymers does not change the resistance behavior of the composite to any great degree, suggesting that particle features of shape, size, arrangement, and so forth are the more important parameters. Such particle morphological details are indicated in the included photomicrographs.