Lubricated squeezing flow rheometry has been used to study the rheological behavior of highly filled polymer composites that stiffen as they are processed. Reactive systems consisting of a crosslinking polymer phase or a polymerizing polymer phase filled with glass beads up to 65% by volume were prepared in a banbury mixer and compared with nonreactive systems of up to 65% filled polydimethylsiloxanes (PDMS) of differing molecular weights. The torque time profile from banbury mixing shows a linear rise in torque with time for the crosslinking system, while the polymerizing system shows a linear followed by power law rise in torque with time. The relaxation modulus following a step strain was measured for the reactive systems at various mixing times. Comparisons of the normalized relaxation moduli of the reactive and nonreactive materials suggest similarities between the effect of filler amount and crosslinking amount and between the combined effects of molecular weight and filler amount and the degree of polymerization. The viscosity in biaxial extension was also measured for each system. The biaxial extensional viscosity behavior of the filled PDMS materials with extension rate was similar to that reported in shear, while the behavior of both of the reactive systems was similar. A plastic-viscoelastic constitutive equation developed by White and Tanaka was used to quantify trends in the viscosity data.