The hypothalamic suprachiasmatic nucleus (SCN) develops as the circadian pacemaker during postnatal life. Although both GABAA and NMDA receptors are expressed in the majority of SCN neurons, postnatal development of their functions has not been analysed. Thus, we studied the receptor-mediated Ca2+ responses in mouse hypothalamic slices prepared on postnatal days (P) 6–16. The NMDA-induced Ca2+ flux was prominent in the SCN and maximal Ca2+ responses in Mg2+-free conditions had no day–night variations in P14–16 mice. At P6–7, extracellular Mg2+ reduced the NMDA-induced Ca2+ flux irrespective of the circadian time whereas, after P9–10, Mg2+ produced a larger reduction at night than during the daytime. Muscimol also significantly increased Ca2+ in the developing SCN. Voltage-sensitive Ca2+ channel blockers inhibited the muscimol-induced Ca2+ increase whereas tetrodotoxin had no effect, suggesting that stimulation of postsynaptic GABAA receptors depolarizes SCN neurons to increase Ca2+. Macroscopic imaging analysis demonstrated a developmental reduction in the muscimol-induced Ca2+ increase preferentially in the nighttime group older than P9–10. The day–night variation in the magnitude of the Ca2+ response was due to two cell populations, one of which exhibited an increase and the other a decrease in Ca2+ in response to muscimol. Because the critical developmental stages for exhibiting day–night variations in the receptor-mediated Ca2+ responses overlapped the maturation of firing rhythms in SCN neurons, the Ca2+ signalling may be necessary for or regulated by the mature circadian clock.