Photomicrography and reflectance microphotometry were used to monitor melanosome movement in frog skin melanocytes in vitro in response to hormonal stimulation and cytochalasin B (CB). Melanocyte-stimulating hormone (MSH), theophylline, and dibutyryl cyclic AMP (DiBcAMP) induced melanosome dispersion (darkening) which was promptly arrested by cytochalasin B in concentrations of 5–20 µg/ml. Melanosome aggregation (skin lightening) occurred only after removal of the darkening agent (MSH, theophylline, or DiBcAMP) and proceeded in the presence or absence of CB. When CB was added to darkened skins, they did not lighten and melanosomes remained in the dispersed state. Use of CB has permitted the dissection of cyclic AMP-mediated melanosome dispersion into two distinct events. The first, induction of melanosome dispersion, is CB sensitive. The second action of intracellular cyclic AMP involves an uncoupling of the centripetal motive force, and is CB insensitive. In the latter process, production of cyclic AMP appears to produce the same result as application of microtubule-disrupting agents.

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