Abstract. The action of gravity stimulation in darkness was examined in agravitropic primary roots of Zea mays L. (cv. Golden Cross Bantam 70). Contents of diffusible and nitric-acid-extractable Ca 2 + in 1-mm apical tips of roots gravistimulated in the dark were measured by flowinjection analysis as free Ca 2+ and bound Ca 2+, respectively. The free-Ca 2+ content increased transiently, reaching a maximum 0.5 h after gravistimulation. This transient increase was also observed when gravistimulation was applied by changing the orientation of the roots back from horizontal to vertical again. On the other hand, the bound-Ca 2+ content decreased transiently following gravistimulation. Furthermore, when the root caps were treated with 10 mM 2-(N-morpholino) ethanesulfonic acid buffer, the elevation of free Ca 2 + following gravistimulation was prevented. These results indicate that gravity perception and the initial transduction steps proceed in the dark, and moreover that the elevation Of free Ca 2+ brought about by the interaction of Ca2+/H + in the apoplast of root tips may be involved in transmission of the gravity signal.