The work described in the present paper was performed to establish stress–strain–time relationships at plastic sheet thermoforming temperatures. The relationships are correlated with sheet-forming “formability”. Specimens of poly(methyl methacrylate) at 165°C and high-impact polystyrene at 122°C were extended to large strains at constant cross-head velocities. Initial strain rates were between 4.2 × 10−3/sec and 1.6 × 10−1/sec. It was found that the flow stress σ was related to the true strain ε and the elapsed time t by a relation σ = Ktmεn, where K is a constant and n and m′ are indices. The value of n for both materials was approximately one. The value of m′ was −0.052 and −0.33 for poly(methyl methacrylate) and high-impact polystyrene, respectively. Tests were also performed in which the cross-head velocity was increased in steps. It was found that the flow stress in these tests followed the same relationship as in the constant cross-head velocity tests.