Circumstantial evidence has led to the theory that substance P (SP) — an undecapeptide discovered over 60 years ago — is involved in the transmission of pain information from the central nervous system. More direct evidence is now provided by two studies of mice in which the function of SP has been genetically disrupted. Both studies find that the animals show reduced responses to painful stimuli, indicating that SP acts as a chemical neurotransmitter or modulator along pain pathways. Moreover, the mice show a reduced stress response when they're dunked in cold water, indicating that SP may also be involved in the response to stress.