The effects of external factors on both H2 production and bidirectional Hox Hoxhydrogenase activity were examined in the non-N2 fixing cyanobacterium Synechocystis PCC 6803. Exogenous glucose and increased osmolality both enhanced H2 production with optimal production observed at 0.4% and 20 mosmol kg-1, respectively. Anaerobic condition for 24 h induced significant higher H2ase activity with cells in BG110 showing highest activities. Increasing the pH resulted in an increased Hox-hydrogenase activity with an optimum at pH 7.5. The Hoxhydrogenase activity gradually increased with increasing temperature from 30℃ to 60℃ with the highest activity observed at 70℃. A low concentration at 100 μM of either DTT or β-mercaptoethanol resulted in a minor stimulation of H2 production. β- mercaptoethanol added to nitrogen and sulfur deprived cells stimulated H2 production significantly. The highest Hox-hydrogenase activity was observed in cells in BG110-Sdeprived condition and 750 μM β-mercaptoethanol measured at a temperature of 70ºC; 14.32 μmol H2 mg chl a-1 min-1. The nitrate assimilation mutant strains ΔnarB, ΔnirA and double mutant, Δnar:ΔBnirA gave higher H2 production than the wild type when cells were adapted in normal BG11 for 24 h. Nitrate uptake in response to osmotic upshifts in the non-diazotrophic cyanobacterium Synechocystis sp. strain PCC 6803 was studied. A small increase of osmolality by 30 and 40 mosmol kg-1 sorbitol and NaCl resulted in about 3.5- and 4.5- fold increase of nitrate uptake, respectively. At 25 mosmol kg-1 or higher, NaCl exhibited higher nitrate uptake than sorbitol suggesting a stimulatory effect of Na+ on the uptake activity. External 20 mM NaCl stimulated nitrate uptake with Ks and Vmax values of 79 μM and 2.45 μmol min-1 mg Chl-1, respectively which were about 2-fold higher than those without NaCl. Ammonium and DL-glyceraldehyde, an inhibitor of CO2 fixation, caused a reduction of nitrate uptake. Cells pre-incubated in darkness showed drastic reduction of uptake activity by 70% suggesting energy-dependent nitrate uptake systems in Synechocystis sp. strain PCC 6803. Nitrate transport was sensitive to various metabolic inhibitors including those dissipating proton gradients and membrane potential. Altogether, the results suggest that nitrate uptake in Synechocystis sp. strain PCC 6803 is dependent on and stimulated by Na+ ions and that the uptake requires energy provided by electrochemical potentials generated by electron transport.