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This work reports the first demonstration that corticosterone (CORT) has a rapid and transient effect on NMDA receptor‐mediated Ca²⁺ signaling in cultured rat hippocampal neurons. Using single cell Ca²⁺ imaging, CORT and agonists of glucocorticoid receptors were observed to modulate the NMDA receptor‐mediated Ca²⁺ signals in a completely different fashion from pregnenolone sulfate. In the absence of steroids, 100 µm NMDA induced a transient Ca²⁺ signal that lasted for 30–70 s in 86.1% of the neurons prepared from postnatal rats (3–5 days old). After pre‐treatment with 0.1–100 µm CORT for 10–20 min, NMDA induced extremely prolonged Ca²⁺ elevation. This prolonged Ca²⁺ elevation was terminated by the application of MK‐801 and followed by washing out of CORT. The proportion of CORT‐modulated neurons within the NMDA‐responsive cells increased from 25.1 to 95.5% when the …