Updated project metadata. Dynamic interactions between RhoA and Rac1, members of the Rho small GTPase family, play a vital role in the control of cell migration. Using predictive mathematical modelling and experimental validation in MDA-MB-231 mesenchymal breast cancer cells, we show that Rac1 and RhoA interactions via the PAK-family kinases can produce bistable, switch-like responses to a graded PAK inhibition. Using a small chemical inhibitor of PAK we confirm the model conjecture demonstrating that cellular RhoA and Rac activation levels respond in a bistable manner to PAK inhibition where for a given inhibition level these levels are high or low depending on the history of the system. Consequently, we show that downstream signalling, actin dynamics and cell migration also behave in a bistable fashion, displaying abrupt switches and hysteresis in response to PAK inhibition. In summary, our results demonstrate that PAK is a critical component in the Rac1-RhoA inhibitory crosstalk that mediates bistable GTPase activity and cell migration switches.