--- - |2+ Key points NMDA receptors (NMDARs) are required for long‐term depression (LTD) at parallel fibre–Purkinje cell synapses, but their cellular localization and physiological functions in vivo are unclear. NMDARs in molecular‐layer interneurons (MLIs), but not granule cells or Purkinje cells, are required for LTD, but not long‐term potentiation induced by low‐frequency stimulation of parallel fibres. Nitric oxide produced by NMDAR activation in MLIs probably mediates LTD induction. NMDARs in granule cells or Purkinje cells are dispensable for motor learning during adaptation of horizontal optokinetic responses. Abstract Long‐term potentiation (LTP) and depression (LTD), which serve as cellular synaptic plasticity models for learning and memory, are crucially regulated by N‐methyl‐d‐aspartate receptors (NMDARs) in various brain regions. In the cerebellum, LTP and LTD at parallel fibre (PF)–Purkinje cell (PC) synapses are thought to mediate certain forms of motor learning. However, while NMDARs are essential for LTD in vitro, their cellular localization remains controversial. In addition, whether and how NMDARs mediate motor learning in vivo remains unclear. Here, we examined the contribution of NMDARs expressed in granule cells (GCs), PCs and molecular‐layer interneurons (MLIs) to LTD/LTP and motor learning by generating GC‐, PC‐ and MLI/PC‐specific knockouts of Grin1, a gene encoding an obligatory GluN1 subunit of NMDARs. While robust LTD and LTP were induced at PF–PC synapses in GC‐ and PC‐specific Grin1 (GC‐Grin1 and PC‐Grin1, respectively) conditional knockout (cKO) mice, only LTD was impaired in MLI/PC‐specific Grin1 (MLI/PC‐Grin1) cKO mice. Application of diethylamine nitric oxide (NO) sodium, a potent NO donor, to the cerebellar slices restored LTD in MLI/PC‐Grin1 cKO mice, suggesting that NO is probably downstream to NMDARs. Furthermore, the adaptation of horizontal optokinetic responses (hOKR), a cerebellar motor learning task, was normally observed in GC‐Grin1 cKO and PC‐Grin1 cKO mice, but not in MLI/PC‐Grin1 cKO mice. These results indicate that it is the NMDARs expressed in MLIs, but not in PCs or GCs, that play important roles in LTD in vitro and motor learning in vivo. - 'The Journal of Physiology, EarlyView. '