When cucumber seeds are placed in a horizontal position for germination, resulting seedlings develop a specialized protuberance, termed the peg, on the lower side of the transition zone between the hypocotyl and the root, due to gravistimulation. On the other hand, in our space-flight experiment of STS-95, cucumber seedlings grown under microgravity conditions developed a peg on each side of the transition zone, suggesting that gravistimulation was required for suppressing the peg formation on the upper side of the transition zone. We have shown that auxin induces peg formation. In addition, the decrease in mRNA accumulation of auxin-inducible CsIAAl gene was detected in the upper side of the transition zone of horizontally-grown cucumber seedlings, whereas the decrease in CsIAAl transcript level was not found in the transition zone of space-grown seedlings. We have therefore proposed that gravistimulation induces the decrement of IAA in the upper side of the transition zone and thus suppresses peg formation. However, the auxin distribution in the transition zone during peg formation is still unknown. To reveal the IAA distribution upon gravistimulation in the transition zone, we are now investigating the auxin distribution by immunofluorescence method using anti-IAA monoclonal antibody and will present our recent results.