Objective    To observe the synaptic ultrastructural changes of cerebellar dentate-interpositus (D-I) nuclei after acquisition of classically eyeblink conditioned responses (CR) in guinea pigs

and to investigate the relationship between synaptic ultrastructural changes and associative learning.     Methods    Eight albino guinea pigs were used as subjects and randomly assigned into 2 groups: paired (n=4) and unpaired (n=4). A 500-ms tone conditioned stimulus (CS) was paired with a 100 ms corneal oxygen-puff unconditioned stimulus (US) in a standard delay paradigm. The synaptic ultrastructure of D-I nuclei was observed by electron microscopy after 6 consecutive daily training sessions

and the relevant parameters were analyzed with software designed for imaging analysis.     Results    The paired animals reached the CR criteria after the 6 consecutive daily sessions

while the unpaired animals exhibited no appreciable number of CR during the 6 consecutive daily sessions; In comparison with those of the unpaired animals

the paired animals showed no significant increases of the synaptic density (P0.05)

but produced the longer synaptic active zone (P0.05) and the thicker post-synaptic density (P0.05).     Conclusion    The acquisition of classically conditioned eyeblink responses is related to synaptic ultrastructural changes of cerebellar D-I nuclei.