Model. The model depicts known and putative signal transduction pathways that might get activated in Dictyostelium cells in response to hypertonicity. In contrast to yeast no osmosensors are so far known in Dictyostelium. The OP1 (o smostress-dependent p athway 1) pathway is under control of the hybrid histidine kinase DokA and leads to elevated cAMP levels thereby activating protein kinase A (PKA). The cGMP pathway, which we named OSP (o smostress-dependent S TATc p athway), leads to the activation and nuclear translocation of STATc. Other components of this pathway are probably Rap1, a guanylate cyclase (GC), GbpC, PkyA and PTP3. Based on differential regulation of putative MAPK components we propose a third signalling branch, which is under the control of a MAPK cascade, similar to yeast and mammals. Genes that were up-regulated in our experiments are in red. PM: plasma membrane; NM: nuclear membrane.