In many electrically non-excitable cells, Ca²⁺ entry is mediated predominantly by the store-operated Ca²⁺ influx pathway. The best-characterised store-operated Ca²⁺ current is the Ca²⁺ release-activated Ca²⁺ current (I _CRAC). It is generally believed that high concentrations of intracellular Ca²⁺ buffer are required to measure I _CRAC, due to Ca²⁺-dependent inactivation of the channels. Recently, we have recorded robust I _CRAC in rat basophilic leukaemia (RBL-1) cells at physiological levels of Ca²⁺ buffering when stores were depleted by inhibition of the sarcoplasmic/ endoplasmic reticulum Ca²⁺-activated adenosine triphosphatase (SERCA) pumps. However, the second messenger inositol 1,4,5-trisphosphate (InsP₃) was not able to evoke the current under such conditions, despite inducing substantial Ca²⁺ release. We have therefore suggested that a threshold exists within the Ca²⁺ stores which has to be overcome for macroscopic I _CRAC to activate. To establish whether this is a specific feature of I _CRAC in RBL-1 cells or whether it is a more general phenomenon, we investigated whether a threshold is also seen in other cell-types used to study store-operated Ca²⁺ entry. In Jurkat-T lymphocytes, I _CRAC is activated weakly by InsP₃ in the presence of low concentrations of Ca²⁺ buffer, whereas the current is large when SERCA pumps are blocked simultaneously, as in RBL-1 cells. Although the electrophysiological properties of I _CRAC in the Jurkat cell are very similar to those of RBL-1 cells, the Na⁺ conductance in the absence of external divalent cations is quite different. Unexpectedly, we failed consistently to record any store-operated Ca²⁺ current in macrovascular pulmonary artery endothelia whereas robust I _CRAC was seen under the same conditions in RBL-1 cells. Our results show that I _CRAC has a similar profile of activation in the presence of physiological levels of Ca²⁺ buffering for Jurkat T-lymphocytes and RBL-1 cells, indicating that the threshold mechanism may be a general feature of I _CRAC activation. Because I _CRAC in pulmonary artery endothelia is, at best, very small, additional Ca²⁺ influx pathways may also contribute to agonist-induced Ca²⁺ entry.