The glutamine synthetase activity in the wild type and high-light-tolerant mutant of Anacystis exhibited differential response to the increasing light intensity (2-40 W/m(2)). As evident from the results, the glutamine synthetase (GS) activity in the wild type is more dependent on respiration, whereas the GS enzyme in the mutant cells derived its carbon and energy from photosynthesis, Further, results revealed that the reduced GS activity in the wild-type cells under the high-light stress was accompanied by high aspartate amino transferase (AST/GOT) activity and low alanine amino transferase (ALT/GPT) activity. On the contrary, high GS activity in the mutant cells was accompanied by low AST/GOT enzyme activity and high ALT/GPT activity. It was inferred that mutant and wild-type cells adapt to the high-light stress by different mechanisms.