In the wallaby, whisker-related patterns develop over a protracted period of postnatal maturation in the pouch. Afferents arrive simultaneously in the thalamus and cortex from postnatal day (P) 15. Whiskerrelated patterns are first seen in the thalamus at P50 and are well formed by P73, before cortical patterns first appear (P75) or are well developed (P85). This study used the slow developmental sequence and accessibility of the pouch young to investigate the effect of nerve lesions before afferent arrival, or at times when thalamic patterns are obvious but cortical patterns not yet formed. The left infraorbital nerve supplying the whiskers was cut at P0 -93 and animals were perfused at P112-123. Sections through the thalamus (horizontal plane) and cortex (tangential) were reacted for cytochrome oxidase to visualize whisker-related patterns. Lesions of the nerve at P2-5, before innervation of the thalamus or cortex, resulted in an absence of patterns at both levels. Lesions from P66 -77 also disrupted thalamic and cortical patterns, despite the fact that thalamic patterns are normally well established by P73. Lesions from P82-93 resulted in normal thalamic and cortical patterns. Thus, despite the wallaby having clearly separated times for the development of patterns at different levels of the pathway, these results suggest a single critical period for the thalamus and cortex, coincident with the maturation of the cortical pattern. Possible mechanisms underpinning this critical period could include dependence of the thalamic pattern on corticothalamic activity or peripheral signals to allow consolidation of thalamic barreloids. © 2006 Wiley-Liss, Inc. Key words: trigeminal pathway; vibrissa; barrel; somatosensory development; whisker folliclesThe rodent whisker-barrel pathway has provided a model system par excellence for studies of normal somatosensory development and function as well as plasticity after lesioning (Miller et al., 2001;Waite, 2004). The pattern of whiskers on the muzzle is replicated in visible and quantifiable patterns at each level of the neuraxis from brainstem to cortex. Moreover, the patterns develop in a temporal sequence from periphery to somatosensory cortex. The whiskers on the muzzle are visible from embryonic day (E) 14 in the rat, with whisker-related patterns appearing first in the brainstem [E20 to postnatal day (P) 0 -1], then the thalamus (P2-3) and cortex (P3-5) (Killackey, 1985). During this developmental period, pattern development is vulnerable to peripheral lesions, such as removal of the whisker follicles or section of their sensory nerve, the infraorbital. Peripheral lesions in the perinatal period cause death of many trigeminal ganglion cells and permanent loss of patterns in the brainstem nuclei.