We have previously shown that mice carrying the K644E kinase domain mutation in fibroblast growth factor receptor 3 (Fgfr3) (EIIa;Fgfr3(+/K644E)) have enlarged brains with increased proliferation and decreased apoptosis of the cortical progenitors. Despite its unique rostral-low caudal-high gradient expression in the cortex, how Fgfr3 temporally and spatially influences progenitor proliferation is unknown. In vivo BrdU labelling now showed that progenitor proliferation was 10-46% higher in the EIIa;Fgfr3(+/K644E) cortex compared with wild type during embryonic day 11.5 (E11.5)-E13.5. The difference in proliferation between the EIIa;Fgfr3(+/K644E) and wild-type cortices was the greatest in the caudal cortex at E12.5 and E13.5. Inhibition of mitogen-activated or extracellular signal-regulated protein kinase (MEK) in vitro at E11.5 reduced the proliferation rate of the EIIa;Fgfr3(+/K644E) cortical progenitors to similar levels observed in the wild type, indicating that the majority of the increase in cell proliferation caused by the Fgfr3 mutation is mitogen-activated protein kinase (MAPK) pathway-dependent at this stage. In addition, elevated levels of Sprouty were observed in the EIIa;Fgfr3(+/K644E) telencephalon at E14.5, indicating the presence of negative feedback that may have suppressed further MAPK activation. We suggest that temporal activation of MAPK is largely responsible for cell proliferation caused by the Fgfr3 mutation during early stages of cortical development.