The threshold of a cutaneous withdrawal reflex, elicited by calibrated von Frey hairs applied to the foot and leg, has been used to study the development of spinal sensory processing in a group of 50 preterm and full-term infants ranging from 27.5 to 42.5 weeks postconceptional age (PCA). Data sets (108) were collected on initial threshold, the effects of repeated innocuous stimuli, the receptive field of the withdrawal reflex, and the effect of a contralateral stimulus. As reported previously (Fitzgerald et al. 1988, 1989), there was a correlation between PCA and initial threshold. The mean threshold at 29 weeks was 0.237 g (S.E.M. 0.042), whereas the mean threshold at 41 weeks was 0.980 g (S.E.M. 0.134). Repeated stimulation with von Frey hairs led to a significant lowering of threshold or "sensitization" of the reflex in infants of up to 35 weeks PCA. Thereafter, the decrease in threshold was not significant, and habituation was observed. From 27.5 weeks PCA, it was possible to elicit the withdrawal reflex from the whole limb as far up as the top of the thigh and buttock. Below 30 weeks PCA, the thresholds within this receptive field were uniform, but after 30 weeks a gradient of thresholds was observed increasing progressively from the sole of the foot towards the knee. The application of a maintained stimulus to the contralateral limb significantly inhibited withdrawal reflex responses to ipsilateral von Frey hair stimulation, across all age bands. These results illustrate postnatal changes in sensory processing within the human spinal cord.(ABSTRACT TRUNCATED AT 250 WORDS)