Zebrafish were exposed to a gold nanorod solution (0.5mM) for 10min. Mucous membranes were collected either on ITO plate or blotting membrane. The adsorption and retention of gold nanorods in mucous membranes were evaluated by mass spectrometry of gold ions desorbed from the nanorods. Nano-rods modified with cationic polymers were retained in mucous membranes for at least seven days, while anionic and nonionic polymer-modified nanorods were desorbed within three days. These results demonstrate the feasibility of using gold nanorods as a fish tag.