We have investigated conditions leading to the degradation of glycinebetaine in Aphanothece halophytica and have shown the activity of betaine-homocysteine methyltransferase (BHMT). The intracellular glycinebetaine level was decreased approximately 50% after 36 h salt downshock from 2.0 M NaCl medium to 0.5 M NaCl medium. A slight additional decrease of glycinebetaine occurred when salt downshock was combined with dark treatment. The omission of carbon and nitrogen sources in the growth medium further decreased intracellular glycinebetaine. The activity of BHMT increased from 0 to 460 nmol h(-1)mg(-1) after 3 h salt downshock. Higher strength of salt downshock resulted in higher activity of the enzyme. Small increase of the enzyme activity was also observed when A. halophytica was deprived of carbon and nitrogen sources in the growth medium.