Abstract: The aim of this study was to investigate the role of K+ and proline in the regulation of NaCl and sucrose caused stresses in the cyanobacterium Nostoc muscorum. Growth of Nostoc muscorum severely impaired under NaCl and sucrose caused stresses. Addition of 1 mM proline in the incubation mixture restores growth of the cyanobacterium under stress conditions. The growth is more pronounced in the presence of both K+ and proline. The inhibitory effect of stress component also recovered in terms of photosynthetic O2 evolution and nitrogenase activity in the presence of K+ and proline. The present observations suggest that K+ and proline in combination support not only growth but also oxygenic photosynthesis and nitrogenase activity under stress conditions. During the initial phase of stress condition uptake of K+ taken place followed by the accumulation of proline. Thus, the accumulation of K+ and proline in the cell cytoplasm act as a primary and a secondary osmolytes in the investigated organism.