The purpose of this study was to determine if MSH, a peptide hormone, injected within a frog skin melanophore could elicit a physiological response, melanosome dispersion. Multibarreled electrodes were used to iontophoretically inject alpha-MSH inside frog skin melanophores of Rana pipiens pipiens. In 46 cells, intracellular MSH was ineffective in producing melanosome dispersion as viewed through the microscope. Because the frog skin is a complex of closely spaced cells, at times the microelectrode may have impaled cells other than melanophores. Therefore, in order to verify that the electrode was in a melanophore and not some other cell type, cAMP, shown to produce melanosome dispersion, was iontophoretically injected to 17 cells, causing the melanosomes to disperse. In these 17 cells, prior injection of MSH caused no dispersion. The response was monitored by observing the target cell with surrounding cells serving as a control. As an additional control to determine if adequate amounts of MSH were released, the electrode was withdrawn from the cell and placed near a group of melanophores, and in all cases the cells close to the electrode tip showed melanosome dispersion after MSH injection. The results of this study remain consistent with the view that MSH receptors in frog skin melanophores are located on the external surface of the plasma membrane, and MSH injected into the cytoplasm of the cell has no short term effect.