Although glutamate is a major excitatory neurotransmitter in the CNS, the specific route of its synthesis is not known. A possible precursor is ornithine, which is known to be converted in vitro to glutamate via the enzyme ornithine amino transferase (OAT). The aim of the present work was to determine whether inhibition of OAT activity in vivo by local injection of L-canaline would affect levels of glutamate, and whether such changes could be related to a specific glutamatergic pathway. A 100 Mug dose of canaline decreased septal OAT activity by 90% in 5 min. Glutamate content decreased in 2 phases with half lines of 7 min and 8 hr. The rapidly turning over pool appears to be neuronal, since it was largely eliminated after lesion of the glutamatergic input to the septum. Studies are now underway of the incorporation of radiolabelled ornithine inot glutamate in vivo. Initial experiments show that a rapid incorporation can be demonstrated.