EXTRACELLULAR events regulate functions in the cell nucleus by means of calcium ions acting through effector enzymes1-5. Recently, the traditional view of the nuclear pore as freely permeable to small ions6,7 has been questioned as a result of reports that nuclear calcium can be regulated independently of cytosolic calcium8-12. We have used confocal microscopy of fluorescent Ca2+ indicators to investigate the Ca2+ dynamics between cytosol and nucleus in neurons. We find that a previously reported amplification of Ca2+ changes in the nucleus13-16 is a measurement artefact. Small changes of cytosolic Ca2+ cause equally rapid changes in nuclear Ca2+, consistent with the free diffusion of Ca2+ through nuclear pores. In contrast, large cytosolic Ca2+ increases (above 300 nM) are attempted in the nucleus. Our results show the nuclear envelope shapes but does not block the passage of Ca2+ signals from cytosol to nucleus.