Capsaicin is frequently used in neurobiological investigations to selectively inhibit response by the primary sensory afferent neurons. The effectiveness of treatment depends significantly on the age of the animals; newborns are both quantitatively and qualitatively more sensitive than adults. In the present study, we used the [3H]resiniferatoxin binding assay to determine whether this different susceptibility to capsaicin between newborns and adult animals may reflect differences either in receptor affinity or density. We report here that whole spinal cord membranes of neonates bound [3H]RTX with similar affinity and positive cooperativity as did the spinal cord membranes from adult animals (Kd values were 24.8 +/- 3.7 and 26.8 +/- 4.8 pM, respectively; Hill coefficients were 2.25 +/- 0.03 and 2.17 +/- 0.05, respectively). However, the receptor density was three-fold higher in the spinal cord membranes of neonates than of adult rats (Bmax values were 142 +/- 13 and 43 +/- 3 fmol/mg protein, respectively). We found no significant difference in the [3H]RTX binding properties of dorsal root ganglia membranes of newborn and adult animals. Our results suggest that a higher density of the vanilloid receptor in the spinal cord (but not in the dorsal root ganglia) of newborn animals may contribute to the quantitative differences between the sensitivity of adult animals and neonates.