Cells which do not contract in response to high-K solution or to receptor activation and which have long thin processes can be found immediately after dispersion of vascular tissues. We have called these interstitial cells (ICs) and in some vessels these cells are positive for the tyrosine kinase c-kit which is usually used as a marker for Interstitial Cells of Cajal found especially in intestinal smooth muscle (Sanders et al, 199; Hirst & Ward, 2003). These ICs appear to be regulating their [Ca2 ]i after enzyme dispersion as imaging [Ca2 ]i using fluo-3 in single ICs revealed spontaneous transient events which were however, generally much longer lasting than calcium events in smooth muscle cells. They were associated with longer lasting outward currents probably caused by the opening of calcium-activated potassium channels. These calcium events also extended to the thin processes of ICs.After enzyme dispersion the processes of surviving ICs from guinea-pig mesenteric artery were observed to grow slowly. This growth was inhibited by latrunculin B which is an inhibitor of actin polymerisation. In enzyme dispersions of rabbit portal vein cells surviving contacts between the processes of single ICCs and the bodies of smooth muscle cells (SMCs) were observed in electron micrographs. These were also detected in living IC-SMC pairs under the optical microscope. Spontaneous rhythmical [Ca2 ]i oscillations were observed in ICs after fluo-3 loading and were associated with depolarisations of membrane recorded by a tight-seal perforated patch technique. To investigate the possibility of signal transmission from IC to SMC, the IC was stimulated under voltage-clamp using perforated patch technique, while changes in [Ca2 ]i in the stimulated cell as well as in an adjacent SMC were monitored using fast x-y confocal imaging of fluo-3 fluorescence. Following stimulation of an IC under voltage-clamp by depolarising steps similar in duration to depolarisations associated with spontaneous [Ca2 ]i oscillations, a transient elevation of [Ca2 ]i in adjacent SMC occurred with a substantial delay (up to 4 s). The role of these ICs in the walls of blood vessels is not clear but they show spontaneous transient changes in membrane potential and [Ca2 ]i which can trigger changes in [Ca2 ]i in smooth muscle cells.