Histochemical and autoradiographic techniques were used to study mossy fiber sprouting which occurs in the hippocampus of kindled rats. In rats kindled by daily stimulation of amygdala or entorhinal cortex, the mossy fibers sprout to innervate the supragranular zone of fascia dentata; this synaptic reorganization was associated with a significant increase in the density of high-affinity kainic acid binding sites. Amygdala but not entorhinal kindling also induces sprouting of mossy fibers in the stratum infrapyramidale of CA3.