The effects of two sulfhydryl inhibitors, Igrosin (containing mersalyl as an effective agent)and N-ethylmaleimide (NEM), were studied on pigment migration in melanophores of Oryziaslatipes. Activities of melanophores were measured photoelectrically and simultaneously confirmedby microscopic observation. Pretreatment with each of these inhibitors at a concentrationof 10^<-3> M blocked completely pigment aggregation within the melanophores to KClor adrenaline. The effects of these agents could not be removed by washing with the physiologicalsolution alone. Cysteine reversed the effects of these inhibitors on the response of melanophoresto KCl or adrenaline. These inhibitors produced dispersion of pigment in punctateadrenaline-treated melanophores which was markedly rapid as compaired with that producedby the saline alone. If one of the inhibitors was applied in combination with adrenalinefollowing temporary pigment aggregation within the melanophores rapid pigment dispersionwas induced. Based on these results, possible involvement of the elements of protein -SHgroups on pigment aggregation mechanism was discussed.