The nucleus accumbens septi (Acb) represents an interface between limbic and motor systems and a site for modulation of these integrative functions by ascending catecholaminergic, principally dopaminergic, axons. This modulatory regulation is most likely attributed to pre- or postsynaptic associations between limbic telencephalic and brainstem afferents. In the present investigation, we examined the ultrastructure and synaptic associations of hippocampal afferents, as well as their relation to catecholaminergic terminals, in the medial Acb of adult rats. Hippocampal afferents were identified by anterograde transport of wheat germ agglutinin-conjugated horseradish peroxidase (WGA-HRP) injected in the ventral subiculum, and by anterograde degeneration seen 2-3 days following lesion of the fimbria. Specific comparisons between these methods were made (1) to determine whether similar populations of terminals were labeled and (2) to assess the feasibility of combining degeneration with immunoperoxidase labeling for the catecholamine synthesizing enzyme, tyrosine hydroxylase (TH). Hippocampal afferents labeled with HRP were finely myelinated or unmyelinated and gave rise to small terminals (mean diameter 0.58 micron) containing mostly clear, round vesicles. Of the HRP-labeled terminals which made recognizable junctions, 85% (104/122) formed asymmetric synapses with the heads of dendritic spines. The remainder either formed asymmetric axodendritic synapses or symmetric junctions. Degenerating terminals were significantly smaller (mean diameter 0.35 micron) than terminals labeled with HRP. However, these also formed principally asymmetric axospinous synapses (89/102, 87%). Whether identified by HRP transport or anterograde degeneration, the hippocampal afferents comprised approximately 10% of all terminals and 30% of all asymmetric axospinous synapses in the medial Acb. In contrast to hippocampal afferents, TH-labeled terminals formed primarily symmetric contacts with dendritic shafts and the heads and necks of spines. Quantitative analysis of sections containing both anterograde degeneration and TH-immunoreactivity showed that 25% (26/104) of associations formed by degenerating hippocampal terminals involved convergent inputs with TH-labeled terminals on the same postsynaptic structure. These included dual input either to the same spine head or to different parts of the same dendrite. In addition, the plasma membranes of hippocampal and TH-labeled terminals were often directly apposed to each other (10/58, 17% of axo-axonal associations formed by degenerating terminals), without recognizable synaptic specializations.(ABSTRACT TRUNCATED AT 400 WORDS)