Twenty-one days after ovariectomy (OVX) an increased frequency of lamellar cytoplasmic organelles, termed 'whorl' bodies (WB), was observed in neurons of the rat arcuate nucleus (AN). When estradiol valerate (2 mg s.c.) was injected either at the time of OVX or one week later, the frequency of WB at 21 days was reduced. The estrogen treatment resulted in a concomitant rise in the frequency of 'nematosomes' (NS), filamentous electron-dense cytoplasmic structures. In the medial part of the AN glutamic acid decarboxylase (GAD) immunopositive symmetric (Gray II) synapses were observed in contact with WB- and NS-containing cells. After colchicine treatment, GAD immunoreactivity was observed in the WB- and NS-containing perikarya in the medial AN. Some of the NS-containing cells in the lateral AN of the colchicine-pretreated animals remained immunonegative for GAD.