Objective  To investigate the role of cerebellar dentateinterpositus (D-I) nuclei during acquisition of classical eyeblink conditioning in guinea pigs.  Methods A 500 ms tone conditioned stimulus (CS) was paired with a 100 ms corneal oxygen-puff unconditioned stimulus (US) in a delay paradigm. Guinea pigs were trained daily to acquire the classically conditioned eyeblink responses. Micorinjections of Muscimol into bilateral cerebellar D-I nuclei were performed during the 4-6 day of training session. A high-resolution potentiometer was used to detect t

he eyeblink responses.  Results The conditioned response (CR) rates of Muscimolinjected group on session 4-6 were significantly lower than their rates on session 3 (P

1

=0.005， P

2

=0.004， P

3

=0.010); Their rates on session 7 and 8 were as the same level as those without microinjection on session 4 and 5 (P

1

=0.061， P

2

=0.669). However

there was no significant difference on their amplitude and peak latency of unconditioned responses (URs) during the entire course of training (P

1

=0.926， P

2

=0.939).  Conclusion The cerebellar D-I nucleus is actively involved in the neural circuitry of motor learning

which is essential for both acquisition and performance of the classically conditioned eyeblink responses.