Endothelial cells play an important role in the vascular responsiveness to many stimuli by releasing locally active agents. The intracellular signal which links the external stimulus to the release of the active compounds is almost certainly an elevation in cytoplasmic calcium (Cai2+). Thus a detailed knowledge of Cai2+ regulation is central to an understanding of the physiology and pharmacology of endothelial cells. The present experiments, on single bovine aortic endothelial cells, demonstrate that agonists stimulate complex changes in Cai2+. These include rapid and regular fluctuations in Cai2+ which are different from the oscillations reported in other endothelial cells and non-excitable cells. The fluctuations are completely abolished in media containing low calcium, 2 mM-cobalt or caffeine but are not affected if the cells are bathed in isotonic potassium solutions. The hypothesis is put forward that the fluctuations in Cai2+ are associated with localized influxes of calcium and are possibly involved with the recycling of calcium between the internal stores, the cytoplasm and the external medium.