An abnormal increase in the number and distribution of smooth muscle cells (SMCs) is a key feature of the vascular remodelling that underlies cardiovascular disease. The predominant explanation for these changes is that medial SMCs undergo phenotypic modulation followed by migration to the intima and subsequent proliferation [1]. However, despite decades of research there has been not been a single direct demonstration of SMC phenotypic modulation [2]. Recently, the very existence of phenotypic plasticity has been questioned [3]. We therefore employed high-resolution, multi-wavelength fluorescence and simultaneous phase contrast, time-lapse microscopy to monitor the fate of unambiguously identified, fully differentiated SMCs. Cells were isolated from the smooth muscle layer of guinea pig portal vein, carotid artery or colon tissue by enzymatic digestion and trituration. The resulting cell suspensions contained both SMCs and a range of other cell types (57±11%, S.D., for colon samples, n=9). The isolated cells were maintained in culture conditions (1:1 Waymouth’s:Ham’s F-12 media containing 10% FBS) within a microscope stage-top incubator and visualized continuously for ~1 week (acquisition rate 0.03Hz).Phenotypic modulation of the initially contractile SMCs (cells were responsive to phenylephrine/carbachol) was clearly observed in response to the serum-containing media (n=30). Extensive cellular remodelling occurred and a clear sequence of events was observed consistently. First, the spindle-shaped SMCs rounded up and remained round for varying lengths of time (typically 1-3 days). Next the cells spread outwards and, at this stage, repetitive contractions occurred (measured via bursts of 10Hz imaging). Following this, the cells began to migrate. Significantly, after the onset of migration, there was clear evidence of phagocytic activity in transformed SMCs. This phagocytic activity was confirmed by the cells ability to internalise 1µm latex beads. Furthermore, the extrusion of subcellular structures from one cell and their subsequent uptake by another cell occurred frequently. There was also repeated formation of highly dynamic direct connections (tunnelling nanotubes) between transformed SMCs and nearby cells. Significantly, whilst other cell types present in isolations from smooth muscle tissue proliferated rapidly to colonize the culture chambers, the SMCs observed did not. This observation may question the origin of cells widely used in ‘SMC cultures’ to study signalling pathways and the role of SMC proliferation in vascular remodelling. These results highlight the complexity of the remodelling process, the highly dynamic cellular interactions involved and the significant cell-cell variation in response and will enable a detailed picture of SMC remodelling to be built.