Regulation of cytoplasmic and nuclear Ca in cardiac myocytes is crucial for regulation of contractile activity and gene expression, respectively. We studied endothelin-1 (ET) and isoprenaline (ISO) induced subcellular changes in electrically evoked Ca transients (CaT) in fluo-4-loaded rabbit atrial myocytes using fast, two-dimensional confocal microscopy. Nuclear CaT were similar in amplitude but significantly slower than cytoplasmic CaT. ET (1–10nM, n=23) increased cytoplasmic CaT to 130% of pre-drug control. The ET-induced increase in nuclear CaT was significantly larger (by 10–15%, P<0.05) than the cytoplasmic CaT increase. Both effects were abolished or attenuated by inhibition of ETA receptors (0.25mM BQ123, n=4), PLC (3mM U73122, n=7), or IP3 receptors (3mM 2APB, n=9; each P<0.05). ISO (2–10nM, n=7) caused similar increases in cytoplasmic CaT (+30%). In contrast to ET, however, the ISO-induced increases in nuclear CaT did not exceed those in the cytoplasm. Furthermore, ISO, but not ET, accelerated relaxation of cytoplasmic and nuclear CaT. Thus, ET elicited larger and longer lasting increases in nuclear Ca than ISO. The results suggest that, depending on agonist and intracellular signalling cascade activated, there may be distinct differences in nuclear Ca signalling. ET enhances nuclear CaT via PLC-IP3 signalling. ISO, though, despite eliciting a similar (cAMP-mediated) increase in cytoplasmic CaT, does not exert an additional increase in nuclear Ca.