A recent study, carried out in the monkey brain demonstrated a hitherto undescribed projection from the lateral to the basal nucleus of the amygdaloid complex. In the present study, we used light and electron microscopic techniques to determine whether a similar connection exists in the rat brain and to define what type(s) of synaptic contacts are produced by fibers of this projection. Injections of the lectin tracer Phaseolus vulgaris leucoagglutinin (PHA-L) were placed into several levels of the lateral nucleus and the distribution of fibers in the basal (basolateral) nucleus was evaluated. All lateral nucleus injections resulted in labeled fibers in the basal nucleus, though the density and distribution of labeled fibers depended on the position of the injection site within the lateral nucleus. In general, the heaviest labeling of the basal nucleus was observed after injections at midrostrocaudal levels of the lateral nucleus, especially when the injection was located ventrally. Fibers originating from cells labeled by these injections were observed throughout much of the rostrocaudal extent of the basal nucleus. Rostrally situated injections resulted in substantially lower levels of labeled fibers in the basal nucleus. Injections placed caudally in the lateral nucleus resulted in light to medium levels of labeled fibers in the basal nucleus; the terminal field in these cases did not extend as far rostrally as after the rostral and midlevel injections. Electron microscopic analysis of PHA-L labeled fibers revealed that they contributed synapses to the basal nucleus. The majority of PHA-L labeled terminals formed asymmetric contacts on dendritic spines or shafts; a smaller number of PHA-L labeled terminals formed symmetrical synapses.