Filaments of Anabaena flos-aquae lost over half of their gas vesicles when exposed to light of high irradiance (> 115 mu-mol m-2 s-1) for long periods (up to 23 h). An investigation using different irradiances showed that the major loss of gas vesicles occurred after prolonged exposure to irradiances exceeding 80-mu-mol m-2 s-1. In a time-course experiment it was found that 56% of the gas vesicles were lost after 16 h exposure to an irradiance of 135-mu-mol m-2 s-1. Most of this loss could be accounted for by collapse under turgor pressure, which rose by 0.24 MPa to 0.54 MPa after 16 h. A further unexplained loss of 15% after 23 h exposure may indicate that prolonged high irradiance can cause weakening of gas vesicles in the cells. In this time-course experiment 98% of the filaments lost their buoyancy after 6 h exposure, before any gas vesicles were lost; this buoyancy change was quantitatively accounted for by the rise in cell carbohydrate. The relative importance of gas vesicle collapse and carbohydrate accumulation as mechanisms of buoyancy loss is discussed in relation to vertical movements of planktonic cyanobacteria in lakes.