The impact of hypergravity and simulated weightlessness were studied to check whether cyanobacteria perceive changes of gravity as stress. Hypergravity generated by a low-speed centrifuge increased slightly the overall activity of dehydrogenases, but the increase was the same for 90 g and 180 g. The protein pattern did not show qualitative alterations during hypergravity treatment up to 180 g. Cells of Synechocystis PCC 6803 subjected to common stressors like salt, heat, and light clearly accumulated at least four general stress proteins (25, 31, 34, and 63 kDa, respectively). Three of these proteins could also be detected after hypergravity, but in such small amounts that their occurrence could only be taken as a weak indication of stress. Low-molecular-weight stress metabolites were not synthesized in response to hypergravity, indicating that this gravity change was unable to activate the osmotic signal transduction chain. Gravity-dependent alterations were observed only during simulated weightlessness (generated by a fast-rotating clinostat). The glutamate/glutamine ratio was significantly shifted toward a higher glutamine portion. Altogether, the results may indicate that moderate changes of gravity were hardly, if ever, sensed as stress by cyanobacteria.