The stress state during "simple" superplastic bulge forming (without a back pressure) is different from that when a back pressure is applied. In the former procedure, specimens or components are deformed under a biaxial tensile stress state, while in the latter, the deformation is achieved under the combination of a biaxial tensile stress and a uniaxial compressive stress state. Both theoretical and experimental studies have shown that when a back pressure is present, the deformation cannot be treated as simply governed by the difference between the forming pressure and the back pressure. The analytical expressions for the forming relationships and the influence of back pressure on experimental m-log epsilon(e) (where m is the strain-rate sensitivity and epsilon(e) is the equivalent tensile strain rate for bulge forming) relationships for Zn-22 wt % Al and Zn-4 wt % Al-1 wt % Cu are given. Results show that with increasing back pressure, the m-log epsilon(e) curve shifted towards higher strain rates, but the maximum m values were not affected.