2-Aminoethoxydiphenyl borate (2-APB) has been widely used as a pharmacological tool in the investigation of calcium signalling mechanisms. Initially described as an IP₃ receptor antagonist, the drug has since been reported to have complex effects on calcium responses in single cells. Of note are its actions on store-operated calcium entry (SOCE); low concentrations (<50μM) enhance, whilst higher concentrations inhibit, SOCE in DT40 cells [1]. To determine if these actions of 2-APB translate to functional responses we studied its effects on contractions of mouse aorta, a tissue in which SOCE has been shown to be important [2]. Aortic rings (3-4mm) from male C57BL6 mice were set up for recording isometric tension changes (resting tension 0.5-0.7g) in Krebs’ solution. Nω-Nitro-L-arginine methyl ester hydrochloride (300μM) and indomethacin (3μM) were included in the Krebs’ solution to inhibit the relaxant effects of nitric oxide and prostanoids respectively. Where thapsigargin (Tg) or noradrenaline (NA) were used as contractile agents the Krebs’ contained verapamil (3μM) to inhibit voltage-operated calcium channels. After 1hr rest (tension adjusted as necessary) rings were contracted by a 3min application of KCl (40mM), repeated every 30min until consistent responses were obtained. Contractions are expressed as mean±SEM (n in parentheses) of the KCl response. Data were analysed by Student’s t test, with p<0.05 considered significant. When increasing concentrations of 2-APB were applied cumulatively to tissues pre-contracted with NA (10μM; n=4), concentrations in the range 0.1 to 3μM produced an increase in tension; concentrations of ≥30μM caused relaxations, with 100μM abolishing the contractile response to NA. Similar results were obtained using Tg (4μM; n=4) as the pre-contractile agent, but with two key differences; first, less than 1μM 2-APB had no effect; second, whilst 100μM 2-APB produced relaxant effects, it did not inhibit the contraction to Tg, but only the enhancement produced by lower concentrations of the drug. 2-APB (1–100μM) had no effect on contractions to KCl (40mM; n=4). In another set of experiments cumulative concentration response curves to NA (1nM–10μM) were constructed before and after treatment with 2-APB (1μM or 100μM). At the lower concentration, 2-APB (n=5) increase both the potency (EC₅₀ control; 52±6.7nM; EC₅₀ in 2-APB 24±4.2nM) and maximum response (control maximum 109.9±14.8%; in 2-APB 133.3±12.7%) to NA. At the higher concentration 2-APB reduced the maximum response to NA (control 81.8±1.85%; in 2-APB 28.7±10.0%) with no change in the EC₅₀ (n=4). These results show that the complex actions of 2-APB in single cells are mirrored by its effects on functional responses in smooth muscle.