The mitogen-activated protein kinase (MAPK) pathway is a highly conserved eukaryotic signalling: cascade that converts extracellular signals into various outputs, such as cell growth and differentiation(1-3), MAPK is phosphorylated and activated by a specific MAPK kinase (MAPKK)(4): MAPKK is therefore considered to be an activating regulator of MAPK. Pmk1 is a MAPK that regulates cell integrity(5) and which, with calcineurin phosphatase, antagonizes chloride homeostasis(6) in fission yeast. We have now identified Pek1, a MAPKK for Pmk1 MAPK. We show here that Pek1, in its unphosphorylated form, acts as a potent negative regulator of Pmk1 MAPK sig-nailing. Mkh1(7), an upstream MAPKK kinase (MAPKKK), converts Pek1 from being an inhibitor to an activator. Our results indicate that Pek1 has a dual stimulatory and inhibitory function which depends on its phosphorylation state. This switch-like mechanism could contribute to the all-or-none physiological response mediated by the MAPK signalling pathway.