A pressure device has been used to obtain information about the forces involved in the maintenance of the aggregated state of melanophores of the angelfish, Pterophyllum scalare. Single aggregated melanophores of isolated scales were submitted to mechanical compression with forces ranging from 50-320 mup. As a function of the gradually increasing force melanophores disperse their pigment, the degree of dispersion being proportional to the intensity of the force. When microtubules are destroyed by treatment with 0.3 mM vinblastine in KCl solution, pigment dispersion in response to the external force is similar to that observed in KCl alone. After changing the medium to NaCl solution, melanin granules remain concentrated in the cell center; the force-induced melanosome dispersion, however, is significantly enhanced. Distinctly lower forces are required to produce an expansion equivalent to that observed in KCl solution. When the medium is changed to vinblastine-KCl again, the dispersion in response to the external force resembles that obtained before NaCl treatment. Removal of Ca++ and Mg++ ions by treatment with 2 mM EDTA or EGTA in Ringer's solution containing 0.1 mM adrenalin produces a remarkable enhancement of melanosome dispersion in response to increasing external force. This effect of EDTA or EGTA is completely reversible. When the medium is changed to normal Ca++-Ringer's, the force/dispersion curve resembles that obtained before EDTA or EGTA treatment. It is concluded that a state of equilibrium exists between the external force and an opposing force produced by the melanophore. The differences in the opposing force under different experimental conditions may be due to a "contractile component". This component seems to be independent of microtubules, as indicated by vinblastine experiments. It "contracts" under aggregating stimulus and "relaxes" under dispersing stimulus. From the data presented in this paper, the order of magnitude of the pressure developed by the contractile component in the completely aggregated state was calculated as between 5-7 p/cm2 in the relaxed state and about 20 p/cm2 during contraction. These values are comparable to those observed in other nonmuscular cells.