Salt tolerance, a multigenic trait, necessitates knowledge about biosynthesis and function of candidate gene(s) at the cellular level. Among the osmolytes, trehalose biosynthesis in cyanobacteria facing NaCl stress is little understood. Anabaena 7120 filaments exposed to 150 mM NaCl fragmented and recovered on transfer to -NaCl medium with the increased heterocysts frequency (7%) over the control (4%). Cells failed to retain Na+ beyond a threshold [2.19 mM/cm(3)(PCV)]. Whereas NaCl-stressed cells exhibited a marginal rise in K+ (1.1-fold) only at 30 h, for Na+ it was 130-fold at 48 h over cells in control. A time-course study (0-54 h) revealed reduction in intracellular Na+ beyond 48 h [0.80 mM/cm(3)(PCV)] suggestive of ion efflux. The NaCl-stressed cells showed differential expression of maltooligosyltrehalose synthase (MTSase; EC 5.4.99.15) and maltooligosyltrehalose trehalohydrolase (MTHase; EC 3.2.1.141) depending on the time and the extent of intracellular Na+ buildup.