Gravity affects growth and morphogenesis in higher plants. Recently, it has become clear that hypergravity induces morphological changes such as inhibition of elongation growth and promotion of lateral growth. Some indirect evidence suggests that changes in the cytoplasmic free calcium concentration ([Ca 2+] c) play an important role in these hypergravity-induced modifications of growth. However, the hypothetical changes in [Ca 2+] c under hypergravity have not been examined. Here, we report the measurement of the [Ca 2+] c changes induced by hypergravity stimuli in Arabidopsis seedlings expressing the calcium reporter, aequorin. When the seedlings are subjected to 20 g-hypergravity produced by centrifugation, [Ca 2+] c transiently increased and decayed exponentially during the hypergravity stimulation. Larger [Ca 2+] c-increase was observed when the magnitude of hypergravity was increased up to 300 g. The [Ca 2+] c-response showed a strong desensitization, and it could not be elicited even 45 min after the cessation of the first stimulation. The [Ca 2+] c-increase was inhibited by externally applied La 3+ or Gd 3+, potential mechanosensitive Ca 2+-permeable channel inhibitors, suggesting that the hypergravity-induced [Ca 2+] c-increase is mediated by the activation of Ca 2+-permeable channels in the plasma membrane.