The origin and function of the interstitial cells of Cajal (ICCs) that are located at the level of the deep muscular plexus (DMP) have not been completely identified. It has been recently reported that these cells express neurokinin-1 (NK1) receptors to which substance P (SP) shows the highest affinity. Studies during pre- and postnatal life have demonstrated that ICCs are identifiable in the rat ileum soon after birth and already show adult features at 7 days of postnatal life. Several neurotransmitters have been identified at the DMP which appear at specific times during development. We have studied the expression of NK1 receptors by ICCs and enteric neurons and the timing of the appearance of SP in the DMP, myenteric plexus (MP) and submucous plexus (SMP) of rat ileum during development. Rats, aged from 18 days of fetal life to adulthood, were used. NK1 receptors and SP were identified by using NK1 polyclonal antibodies and tachykinin (SP/TK) polyclonal antibodies, respectively. NK1-immunoreactivity (IR) was detected in the ICCs immediately after birth and reached maximal intensity at 7 days. From birth, SP/TK-IR fibers originated from short excitatory neurons at the MP and reached the DMP at 1 week of postnatal life. NK1- and SP/TK-IR appeared in the MP neurons in the fetus and in the SMP neurons at weaning. The present study demonstrates that by the first days of postnatal life, the NK1-IR might be used as a marker of the ICCs at the DMP and suggests that these cells may participate in the actions exerted by tachykinins on muscle cells.