Amitotically activated mitogen-activated protein kinase 1 (MEK1) fragments the pericentriolar Golgi stacks in mammalian cells. We show that activated MEK1 is found on the Golgi apparatus in late prophase. The fragmented and dispersed Golgi membranes in prometaphase and later stages of mitosis do not contain activated MEK1. MEK1-dependent Golgi complex fragmentation is through activation by RAF1 and not MEK1 kinase 1. We propose that a RAF1-dependent activation of MEK1 and its presence on the Golgi apparatus in late prophase is required for Golgi complex fragmentation.