Space flight and clinostat experiments have been used to alter the influence of gravity. Lower and higher plants, both specialized and non-specialized to gravity percep-tion, have shown through changes in ultrastructure and metabolism (including the intracellular calcium balance) that cells are gravisensitive (Claasen and Spooner, 1994; Halstead and Dutcher, 1987; Kiss, 2000; Kordyum, 1997). In the presented paper, an attempt was made to summarize some experimental data and concepts con-cerning certain cell gravity-sensing systems in the gravi-tational field and their interactions with the changed environment of microgravity basing on the cytoskeleton behavior and Ca 2+ signaling in altered gravity. It was proposed that a distinction be made between cell gravisensing and cell graviperception. Gravisensing is related to cell structure and metabolism stability in the gravitational field and their changes in microgravity. Graviperception is related to the active use of a gravi-tational stimulus by cells, which are specialized to grav-ity perception, for realizing normal plant orientation in space, for growth and vital activity (gravitropism, gravitaxis) (Kordyum and Guikema, 2001). The structure of graviperceptive cells is diverse, but gravisensors are well known. These are statoliths of different types that change their position in the direction of the gravity vector and thus initiate the next steps of the gravitational response. The structural and functional organization of graviperceptive cells is determined gen-etically. In most cell types not specialized for perception of gravity, the primary sensors are not clearly defined. The idea of positional homeostasis (Nace, 1983) was the first to focus attention on the role of the cytoskeleton in cell graviresponse; it also explained the fixed stable position and optimal cell orientation in the gravitational field as a state of mechanical stress of the cytoskeletal elements and those that maintain cell membrane integ-rity. The cytoskeleton is also considered as an integral unspecialized cell gravireceptor (Tairbekov, 1990). A significant role in stability of the cell's spatio-temporal organization in the gravitational field and its gravisens-ing has been attributed to the cytoskeleton in some other concepts on cell gravisensing. These include static stimu-lation (Sievers et al., 1991), passive gravistimulation (Barlow, 1992), protoplast pressure (Wayne et al., 1990), putative tensegrity (Ingber, 1993), and restrained gravi-sensing (Baluska and Hasenstein, 1997). The cytoskel-eton is known to participate in cytoplasmic streaming and cell organelle motion, mitosis, cytokinesis, endo-and exocytosis, as well as in intracellular transport of substances—all activities that are potentially gravity-sensitive through the cytoskeleton (Cipriano, 1993). The intracellular cytoskeleton, the extracellular matrix, and the cytoplasmic membrane are assumed to represent compartments of sufficient macromolecular organiz-ation to be sensitive to gravity-induced phenomena (Claasen and Spooner, 1994). The cytoskeleton and extracellular matrix are indispensable for cellular and developmental processes that are directly or indirectly linked through the cellmembrane acting as an inter-mediary. The most useful models for investigating cell gravi-sensitivity in space flight or on the clinostat are gravi-perceptive cells with statoliths, e.g. root cap statocytes, alga Chara rhizoids, apical cells of moss protonema. The apical and subapical zones of Chara rhizoids contain thin bundles of microfilaments and the basal zone con-tains thicker ones. The gravitropically responsive apical part contains statoliths—compartments filled with crystallites of barium sulfate (Sievers et al., 1991). It was concluded that although the arrangement of micro-tubules is essential for polar cytoplasmic zonation and the functional polar organization of the actin cyto-skeleton, it is not involved in the primary events of * Tel: +38 044 212 3236; fax: +38 044 212 3236.