Oxytocin (OT) is involved in all aspects of human reproduction. It often triggers an oscillatory [Ca2+]i response which depends on Ca2+ release from the endoplasmic reticulum (ER) and Ca2+ entry from extracellular space. The latter process is vital for replenishment of the ER Ca2+ store necessary for maintaining [Ca2+]i oscillations over time. Both, store-operated and receptor-operated Ca2+ entry pathways have been implicated in the OT-induced [Ca2+]i oscillations [1]. The contribution of the Na+/Ca2+ exchange and the role of intraluminal ER Ca2+ buffering in OT-induced [Ca2+]i oscillations have not been described. We used experimental and theoretical approaches to investigate the role of the above two processes in OT-induced [Ca2+]i oscillations. Methods: Mathematical modelling of the OT oscillator was performed using a modified luminal calcium-sensitive DeYoung-Keiser IP3-receptor model implemented in Matlab and XPPAUT. The experiments were performed on Chinese Hamster Ovary cells stably transfected with human oxytocin receptor (CHO-OT1) using Fluo-4 and wide-field fluorescence microscopy. The following experimental manoeuvres were designed to affect the filling state of the ER: (i) manipulation of [Ca2+]i removal on the Na+/Ca2+ exchanger by removing and re-introducing extracellular Na+ (isosmotic substitution with sucrose), (ii) manipulation of ER Ca2+ level using membrane permeable low-affinity Ca2+ buffer TPEN, (iii) low dose of reversible SERCA pump inhibitor cyclopiazonic acid to slow down the rate of ER Ca2+ uptake. Results: Application of 1nM OT induced sustained [Ca2+]i oscillations in the majority of cells. Na+ removal from the extracellular space caused moderate increase in basal [Ca2+]i and substantially potentiated OT-induced oscillations (typical trace illustrated in Fig.1A). Activation of Na+/Ca2+ exchange by re-introducing the sodium ions into extracellular medium markedly decreased the amplitude and frequency of the OT induced [Ca2+]i oscillations (Fig1B). TPEN (50 μM) substantially reduced the frequency and amplitude of oscillations (Fig 1C). Higher concentrations of TPEN abolished the oscillations. Application of 100nM CPA did not change basal [Ca2+]i but dramatically increased the time delay between initial peak and the onset of oscillations. The frequency of subsequent peaks was not different from that in control (Fig. 1D). Higher concentrations of CPA (0.5 – 10 μM) abolished the oscillations and substantially increased base-line [Ca2+]i. Mathematical model captured these observations when sensitivity of the IP3 receptor to intraluminal Ca2+ and variable Ca2+ flux from outside the cell were introduced. Conclusion: The Na+/Ca2+ exchanger plays important role in establishing the sustained [Ca2+]i oscillations in response to OT. Parameters of these oscillations are extremely sensitive to Ca2+ buffering in the ER.