The development of function was studied in the rat suprachiasmatic nucleus (SCN) by analyzing the rate and pattern of neuronal discharge in vitro from embryonic day 22 (E22) to postnatal day 14 (P14), and comparing these with adult SCN. The firing rate of SCN neurons on E22 is low but there is a clear circadian rhythm with subjective day values (1.6 +/- 0.2 impulses/s) significantly higher than those for subjective night (0.9 +/- 0.2 impulses/s). At E22 most neurons show an irregular firing pattern. The firing rate of SCN neurons gradually increases from E22 to P14 with emergence of patterns of firing of individual neurons that approximate those of adult SCN. In calcium-free medium the firing rate of SCN neurons at P1 is reduced and the circadian rhythm in firing rate is abolished. These results demonstrate that SCN neurons exhibit a circadian rhythm in firing rate early in development, largely prior to the formation of synaptic contacts within the SCN, and indicate that expression of the rhythm requires the presence of calcium ions.