To clarify possible glycinergic transmission in the cerebellum, principal neurons in deep cerebellar nuclei (DCN) of sliced cerebella (200 µm in thickness) from rats (aged 2-14 days) were studied using whole-cell patch clamp techniques. When glycine (100 µM) was applied to the DCN neurons from a "Y tube", large outward currents were induced (average peak amplitude of about 600 pA at -40 mV). The currents were blocked by strychnine (1 µM) and showed reversal potential of -62 mV, which was approximately the estimated Cl- equilibrium potential. The dose-response relation of the currents showed an apparent dissociation constant of 170 µM for glycine and Hill coefficient of 1.6. In the presence of 6-cyano-7-nitroquinoziline-2, 3-dione (CNQX), D-(-)-2-amino-5-phosphonovaleric acid (APV) and bicuculline, which antagonize AMPA-, NMDA- and GABA_A-receptors, respectively, postsynaptic currents sensitive to strychnine (1 µM) were induced in DCN neurons by external perfusion of 20 mM K⁺ saline. Electrical stimulation of surrounding tissues in DCN evoked definite inhibitory postsynaptic currents (IPSCs) in these neurons. The IPSCs had reversal potential of -62 mV and showed sensitivities to strychnine and tetrodotoxin. Thus, the present study has revealed that strychnine-sensitive glycine receptors are expressed in neurons of the DCN of rats and that glycinergic transmission mediated by these receptors is functional in these neurons from stages immediately after birth. The glycinergic innervations are presumably supplied by small interneurons located in the DCN.