The rheological properties of calcium carbonate-filled polypropylene has been examined using a Rheometrics dynamic analyzer RDAII. The study included a steady shear test, a transient stress growth test, and a dynamic oscillatory shear flow. Yield behavior was observed in all kinds of rheological tests for highly filled compounds when the volume loading exceeded a critical value at about 20%. The empirical Cox-Men rule, which is usually applicable to an unfilled polymer, was found to be invalid for highly filled compounds. The modified Cox-Men rule, in which the apparent viscosity versus the shear rate is equal to the complex viscosity versus the frequency-amplitude in the nonlinear region, was found to be valid only for highly filled compounds. The viscosity and the apparent yield values appear to increase with increasing volume loading of filler particles. The surface treatment of fillers, which presumably reduces the interaction between filler particles and the extent of agglomeration, results in major viscosity reductions and decreases in apparent yield values. The yield values determined from various tests are not the same. The results are interpreted in terms of a system forming a filler network due to weak inter-particle forces. The yield stress resulting from the breakdown and recovery of the network is thus dependent on the characteristic time of the individual test.