The unicellular halotolerant alga Dunaliella salina had the ability to oxidize NADH and reduce Fe(CN) 6 3-. The redox reactions were to some extent stimulated by slight hyperosmotic shock (2.0 mol/L → 2.6 mol/L NaC1), but markably inhibited by abrupt hyperosmotic shock (2.0 mol/L → 3.5 mol/L NaC1) and hypoosmotic shock (2.0 mol/L → 1.0 mol/L NaC1; 2.0 mol/L→0.67 mol/L NaC1).With the adaptation of algal cells to osmotic shock by accumulating or degrading intracellular glycerol, the plasmalemma redox activities were also restored.