Norepinephrine (NE) is a central and peripheral neurotransmitter. The cell bodies of most NE neurons in the brain are located in the brain stem, but project throughout the central nervous system where they are especially important in arousal and attention. NE is also the principal neurotransmitter of the sympathetic nervous system in the periphery (the body), a system critical in arousal and which is closely integrated with the central NE system. Norepinephrine is produced from the essential amino acid tyrosine after ring and side-chain hydroxylation and decarboxylation. Measurement of NE and a range of related compounds in cerebrospinal fluid, plasma, and urine have indicated that global basal function of central and peripheral NE appears unaltered in autism; however, the NE systems may be hyperresponsive to stress.