Conditioned inhibition (CI) of the rat eyeblink response and the neuronal correlates of CI in the cerebellar interpositus nucleus were examined in the present study. In Experiment 1, CI was established with a novel, 3-group design. In Experiment 2, neuronal activity in the anterior interpositus nucleus was recorded during CI training and testing. Each rat was given 2 training phases and then tested for CI with summation and retardation tests. Rats given CI training showed behavioral inhibition compared with rats in 2 control groups. Neuronal activity in the anterior interpositus nucleus correlated with behavioral responding during discrimination training and during the summation test. The results suggest that neurons in the cerebellar anterior interpositus nucleus may participate in the acquisition or expression of CI.