The spreading of trypsinized XTH-2 cells (a line derived from Xenopus laevis tadpole heart endothelia) on glass was investigated. Three phases can be distinguished: (1) blebbing of rounded cells, first attachment to a solid substratum and formation of a broad smooth contact area; (2) organization of a peripheral zone of actin fibrils and reinforcement of the basal cytoplasm by a stress fibre-like pattern; (3) extension of lamellae. The first phase seems to be independent of a supply of metabolic energy, while the others clearly depend on it. This is concluded from the close relationship between cellular projection area and energization of mitochondria as revealed by (a) the fluorescence intensity of cells vitally stained with the mitochondria-specific fluorochrome DASPMI (2–4-(dimethylamino)-styryl-1-methylpyridinium-iodine); (b) the degree of spreading in the presence of inhibitors of respiration; (c) effective amelioration of spreading (phases (2) and (3] under conditions of high ATP content. In phase (2) the extension of the central part of the cells becomes stabilized, the cell body settles on the basal cytoplasmic layer and further expansion of the projection area is achieved by lamella formation (phase (3]; motile and stabile regions of the cells become separated. This sequence of events is interpreted as a self-organizing process based on the development of internal hydraulic pressure, actin polymerization and contraction of the newly developed actomyosin network. During trypsinization, depolymerization of actin does not occur but rather on addition of Ca2(+)-containing media. Cellular ATP content drops as well on trypsinization, as on addition of Ca2+. Manganese promotes spreading by decreasing F-actin disassembly and maintaining a high level of cytosolic ATP, most probably because it is not accepted by the calcium pumps. Regarding the association of glycolytic enzymes with F-actin and their influence on actin assembly, lactate dehydrogenase has been inhibited with oxamic acid. This treatment improves the correlation between F-actin content and the degree of spreading; however, the total amount of F-actin remains smaller and the cells spread more.