The lateral reticular nucleus (LRN) in the medullary reticular formation is a major source of mossy fibre afferents to the cerebellum. We have previously shown that many LRN neurones fire extremely regularly and respond to peripheral sensory stimulation with an initial brief pause in firing followed by a resumption of spiking with ‘reset’ timing (Xu et al., 2012). Here we have investigated the morphology and physiology of rat LRN neurones. Whole-cell patch-clamp recordings were made from 18 LRN neurones in brain slices prepared from rats aged postnatal day (P) 5 to P15 (decapitated under isoflurane anaesthesia), using a K+-based intracellular solution with biocytin. Neuronal morphology was deduced from 9 of these LRN neurones in which biocytin labelling was recovered. Of these, 8 had a morphology consistent with ‘triangular’ cells (Kapogianis et al., 1982). These triangular neurones showed spontaneous firing (mean frequency, 3.1 ± 0.5 Hz; CV of inter-spike intervals, 0.17 ± 0.04) and a hyperpolarisation-activated Isag (mean amplitude, 63.7 ± 55.6 pA). In 6 of the 8 triangular cells, glycine was applied via a picospritzer (1 mM, 0.1-1 s, 10 p.s.i) and evoked currents that reversed at -38.5 ± 17.5 mV. These triangular neurones in the LRN may be the source of spontaneously firing mossy fibre inputs to the cerebellum.