When fibroblasts first emerge from explants of embryonic chick heart, most mitochondria are clustered tightly around the nucleus, with very few extending towards the leading lamella. Although cytoplasmic microtubules are well displayed, mitochondria do not obviously codistribute with them. As the fibroblasts cease locomotion and adapt to growth, however, the mitochondria become dispersed through the cytoplasm and clearly codistributed with the microtubules and centrioles but not with microfilament bundles or 10 nm filaments. This rearrangement occurs in concert with the other changes we have shown previously - including the development of pronounced microfilament bundles and of stable and well-defined focal adhesions - and appears to be related to changes in the motility status of the cells rather than to alterations in growth or synthetic capability. Mitochondrial mobility is strongly reduced by the actions of both colchicine and dihydrocytochalasin B showing that orientation and translocation depend on a co-ordinate interaction of microtubules and microfilamentous meshwork around the centrioles as origin. The Golgi apparatus and endoplasmic reticulum do not rearrange dramatically during the phenotypic conversion, although the locomotory cells are characterized by a zone close to the leading lamella, which is completely free from these organelles. The form and distribution of the Golgi apparatus, but not the endoplasmic reticulum, was sensitive to microtubule disruption but was also shown to have direct functional associations with the centriolar region. The relative distributions of the three types of organelle during the phases of cell movements and cell growth, are consistent with their biochemical functions in cellular activity.