Glutamine synthetase (GS) activity of Schizosaccharomyces pombe 972 was high in ammonia-limited cultures, low in phosphate- and sulphate-limited cultures and not detected in glucose-limited cultures. When ammonia was 'pulsed' into an ammonia-limited culture then GS activity decreased at a rate faster than that calculated if enzyme synthesis ceased and enzyme was diluted out by growth. Enzyme activity increased in ammonia-starved, phosphate-limited cultures and in the ammonia 'pulse' system when the added ammonia had been utilised. These increases in enzyme activity were prevented by the presence of 100 mug/ml cycloheximide. GS activity was inversely related to the intracellular concentration of glutamate.