Experiments with skin pieces revealed that the sympathetic nervous system controls the activity of the light-sensitive iridophores in the stripes of the neon tetra. The spectral peak reflected from the cells was shifted toward longer wavelengths as a result of a direct interaction between norepinephrine and alpha-adrenoceptors present on the cell membrane. Adenosine accelerated the recovery from the effects of the amine. Such regulation seems to operate when fish are in an excited state or under stress. Since alpha-melanophore-stimulating hormone (alpha-MSH), melanin-concentrating hormone (MCH) and melatonin caused the responses only at high concentrations, it is possible that these peptides and amine do not affect the properties of the light-reflecting cells in vivo.