Heparin is an effective competitive antagonist of inositol 1,4,5-trisphosphate receptors (IP(3)Rs). It binds to IP(3)Rs and affects calcium homeostasis. Ultra-low-molecular-weight heparin (ULMWH) is heparin's derivative, the present study was designed to test the effects of ULMWH on intracellular calcium concentration ([Ca(2+)]i) in primary cultured neurons. [Ca(2+)]i was measured by Multilabel Counter Victor-1420 using Fura-2/AM as the calcium fluorescent probe. The results indicated that ULMWH decreased the resting [Ca(2+)]i with or without extracellular Ca(2+). They had no effects on high K(+)-induced elevation of intracellular Ca(2+) level indicating that ULMWH had no effect on external Ca(2+) influx mediated by voltage-dependent calcium channels. However, they partially reduced the increase in [Ca(2+)]i induced by glutamate. Furthermore, ULMWH significantly inhibited the inositol 1,4,5-trisphosphate (IP(3))-induced increase in [Ca(2+)]i both in cellular and subcellular level. These results suggest that ULMWH may reduce [Ca(2+)]i in neurons through suppressing Ca(2+) release from IP(3)-sensitive stores.