The subcellular location of mitochondria controls cell function via changes in ATP production and Ca2+ handling. For example, in smooth muscle, mitochondria regulate the Ca2+ signals (Ca2+ puffs) which arise from the inositol-trisphosphate-sensitive channels (IP3R), by operating within an IP3R cluster. Since Ca2+ puffs are rapid transient events, a close apposition of IP3R and mitochondria is required for this control to take place which is incompatible with rapid free movement of mitochondria observed in several cell types. We find mitochondria in fully differentiated smooth muscle cells to be individual, ovoid, immobile structures which lack directed motion. Brownian-like movement was observed but was so restricted in nature that a displacement of individual organelles did not occur. On the other hand, in proliferative smooth muscle mitochondria are much more diverse and exist as small spheres, rod-shapes, filamentous threads and networks. In addition the organelles undergo almost continuous changes in shape and position in the form of both long distance and complex local movements. In intact resistance arteries studied at physiological pressures in vitro, mitochondria are largely stationary, but in a small number of cells mitochondria display mobility like that observed in single proliferative smooth muscle cells. When smooth muscle in the intact resistance artery is encouraged to proliferate, in organ culture, mitochondrial mobility increases dramatically. Significantly, when mitochondrial mobility is reduced pharmacologically smooth muscle proliferation decreases. Mitochondrial dynamics may be required to facilitate the equal segregation of mitochondria into daughter cells during cell division. The changes in mitochondrial dynamics as smooth muscle enters a proliferative state may offer a target for altering proliferation in vascular disease.