There is evidence to suggest a lack of inhibitory control in developing spinal sensory pathways: excitatory cutaneous receptive fields are large and mechanical thresholds low in the first weeks of life (Fitzgerald, 2005). Since robust GABA_A receptor-mediated inhibition of receptive fields is evident by the third postnatal day (P3) (Bremner et al. 2005), we hypothesized that developmental differences in dorsal horn inhibition may occur at the network level. To test this, we studied the spatial organisation of inhibitory receptive fields in adult and neonatal spinal cord. Anaesthesia was established in adult Sprague Dawley rats (~180g) with Hypnorm (0.6 mg/kg; i.p.) and Diazepam (2.5 mg/kg; i.p.) and in neonates (P3) by cooling on ice. The trachea was cannulated and a midcollicular decerebration performed; anaesthesia was then withdrawn. Neuromuscular blockade was induced with pancuronium bromide (2 mg/kg) and animals ventilated with O₂ and spinalised. ECG was monitored throughout. Extracellular spikes were recorded from single dorsal horn cells of the lumbar spinal cord. Excitatory receptive fields (RFs) were mapped with brush and pinch of the ipsilateral plantar hindpaw and inhibitory fields were mapped on the contralateral paw. Pinch of the contralateral plantar hindpaw inhibited spontaneous or evoked activity in 23/29 adult cells and 20/29 P3 cells. Strength of inhibition was quantified as the number of spikes inhibited relative to the baseline rate of firing. For each cell, the contralateral paw area that produced the maximal inhibition was established and the remaining hindpaw areas were mapped according to whether they produced <20%, 20-39%, 40-59%, 60-79% or ≥80% of the maximal inhibition. Data are presented as mean ± SEM. The spatial organisation of the inhibitory RFs differed significantly at the two ages. In the adult, the strongest (≥80%) inhibition covered 27.7 ± 5.3% of the plantar surface and was restricted to the toes and pads in 17/21 cells. In contrast, at P3, inhibition was more evenly distributed, with pinch to large regions of the paw often inhibiting firing equally strongly: the strongest inhibition covered 69.1 ± 6.5% of the plantar surface and was restricted to the toes and pads in only 2/17 cells. Plotting the strength of inhibition against cumulative plantar coverage and measuring the area under the curve (AUC) for each cell revealed a highly significant difference between the two ages (adult: 5388 ± 318; P3: 7223 ± 298 (Mann Witney p < 0.001), where 8000 represents equal inhibition across the whole plantar area). These results demonstrate substantial postnatal changes in the spatial organisation of inhibition in the developing dorsal horn. Contralateral inhibition is present at P3 but the area of maximal inhibiton is less spatially restricted than in the adult. This lack of inhibitory tuning may contribute to the observed excitability of neonatal cutaneous sensory processing.