Tensile stress-extension ratio curves up to material failure were generated from the test data at constant strain rates ranging from 0.1 to 229 s-1. Several transitions associated with stain-induced crystallization were observed in both materials. Transitions occurred between 0.1-110 s-1 and above 110 s-1 in the unfilled natural rubber, and for the 25 phr carbon black-filled natural rubber transitions occurred between 0.1-132 s-1 and beyond 132 s-1. The unfilled natural rubber became more compliant when the strain rate increased from 110 to 206 s-1, and it became stiffer when the strain rate increased from 206 to 229 s-1. The filled natural rubber became stiffer when the strain rate increased from 83 to132 s-1, and it became more compliant when the strain rate increased from 132 to 194 s-1.