We used a fluorescent dye, FM1-43 to investigate mechanotransduction mechanisms in the hair cells of lateral line organs of Xenopus larvae. FM1-43 specifically labeled the hair cells. The photo-oxidation technique was performed with election microscopy to examine the labeling sites and their mechanisms. The results showed that the labeling sites were mitochondria and rough endoplasmic reticulum throughout the cytoplasm. Endocytic activity of the hair cells was limited to endosomes and small granules located at the apical part of the cells. Blockers of the mechanosensitive cation channel (neomycin, gentamicin, streptomycin, and amiloride) effectively inhibited FM1-43 labeling. One of the blockers, amiloride, was found to bind to hair cells when its fluorescence was examined. Divalent cations such as Mg2+ and Ca2+, but not monovalent cations such as Na+ and K+, inhibited FM1-43 labeling when they were added in excess amounts. These results suggest that FM1-43 internalizes hair cells via the putative mechanosensitive cation channel in the plasma membrane.