The suprachiasmatic nucleus (SCN) of male rats was estimated contain 16 X 10(6) synaptic appositions (unilaterally) or 250 X 10(6) appositions in 1 mm3 tissue of the nucleus with an average of 1404 appositions per neuron. There are significantly fewer synaptic appositions in the suprachiasmatic nucleus of female rats (15 X 10(6) per SCN or 236 X 10(6) in 1 mm3 tissue of SCN with 1264 appositions per neuron on an average). Additionally, numbers of various types of synapses (axo-somatic, invaginated, dendrodendritic and optic) are estimated.