The fine structure of nerve fibers with corticotropin-releasing factor (CRF)-like immunoreactivity in the central amygdaloid nucleus and CRF-containing afferents to the nucleus were investigated by pre-embedding immunoelectron microscopy and by the combination of fluoro-gold tracing and the indirect immunofluorescence method. Significant numbers of CRF nerve endings and dendrites formed synapses with non-immunoreactive dendrites and axon terminals, respectively. Axon terminals devoid of CRF frequently made synapses with the soma of immunoreactive and non-immunoreactive neurons; CRF nerve endings in contact with the soma were fewer in number. Occasionally, CRF was localized to both pre- and postsynaptic structures in the central amygdaloid nucleus. After fluoro-gold injection into the central amygdaloid nucleus and adjacent areas, double-labeled cells with the tracer and CRF were observed mainly in the lateral hypothalamic area and occasionally in the dorsal raphe nucleus, and they were less numerous than single-labeled cells. These findings suggest that part of the CRF axon terminals identified in the electron micrographs arises from neurons in the lateral hypothalamic area and the dorsal raphe nucleus and the others from intra-amygdaloid CRF neurons. The immunoreactive dendrites are likely to derive from neurons in the central amygdaloid nucleus, which are shown to send axons to the lower brainstem. Thus, this study demonstrates that CRF structures constitute a more complex neuronal network in the central amygdaloid nucleus than previously considered.