1. Neural regulation of the density of sodium (Na+) channels in rat muscle was studied by measuring specific binding of tritiated saxitoxin ([3H]STX) to muscles from rat hindlimbs during normal development and in rats in which neuromuscular function was interrupted by sciatic nerve section or neuromuscular blockade with botulinum toxin (BoTX). 2. The normal developmental increase in [3H]STX binding site numbers followed a simple exponential with a time constant of 12 days. The most rapid incorporation of channels coincided with the onset of accelerated muscle growth and increased neuromuscular activity at 2 weeks of age. 3. Elimination of neuromuscular activity retarded muscle growth and inhibited the normal incorporation of Na+ channels into neonatal muscle. Muscles denervation was more effective than BoTX paralysis: denervation at 2 weeks of age prevented the normal 3-fold increase in the binding site density between 2 and 3 weeks of age while age-matched BoTX-treated muscles incorporated an average of 66% of the normal Na+ channel incorporation. 4. Denervation and BoTX treatment were equally effective in reducing the numbers of [3H]STX binding sites in adult muscle. A reduction of 30% in binding sites brought the numbers to levels which corresponded with levels normally seen in muscles at 3 weeks of neonatal development. 5. It was concluded that the neural influence on incorporation of Na+ channels into membranes of neonatal muscle is, at least in part, mediated by neuromuscular activity.