(1) Semi-quantitative studies on synaptic structures in the arcuate nucleus (ARCN) of female rats were performed by counting synapses in 18,000 μm 2 area in the middle part of the ARCN in each brain. (2) In 5-day old rats, the neuropil was in an extremely immature state, which was characterized by the presence of extensive extracellular space. Axodendritic and axosomatic synapses at this age were very few in number. In 20-day old rats, the extracellular space was fully occupied by axons, their terminals, dendrites and glial processes. The numbers of axodendritic and axosomatic synapses were increased to more than four and five times those of 5-day old rats, respectively. (3) At 31 days of age, a further increase in the number of synapses was observed. Although the neuropil still contained a mixed population of morphologically immature and mature synapses at this age, the ultrastructural features of the ARCN approached closely those of the adult. (4) The mean numbers of axodendritic and axosomatic synapses at 150 days of age were more than 16 and 17 times those of 5-day old rats. (5) In 6-day old rats which had been treated with estradiol benzoate (EB) from days 1 to 5, the extracellular space in the neuropil matrix was considerably reduced, compared with that of the control. (6) This tendency was much pronounced in 11-day old rats injected with EB from days 1 to 10. (7) In 31-day old rats receiving EB injections for the first 30 days of life, the mean number of axodendritic synapses in the same field area of the ARCN was 2.7-fold greater than that of the controls. As for the axosomatic synapses, there was no significant difference between the estrogenized and controls. The morphologically mature synapses were much more frequently encountered in the estrogenized rats. (8) At 150 days of age, axodendritic synapses did not show a further increase in number and the mean number was not significantly different from those of 31-day old estrogenized and 150-day old control rats. The mean number of axosomatic synapses was also not significantly different from that of the controls. (9) These results suggest that estrogen may have a facilitatory influence on the development of synaptic structures in the ARCN when it is given early postnatally.