Substance P (SP) administered subcutaneously to neonatal rats on days 1-7 after birth produced long-term physiological changes. The changes included altered pain perception and increased sialogogic response to SP, although the hypotensive response to SP was unchanged. Early exposure to the peptide therefore influenced development, particularly with respect to two systems in which SP is physiologically active. Both changes may reflect an increased sensitivity of these systems to the effects of SP.