Abstract 1. Male Wistar rats were injected i.p. with DL- m-tyrosine (150 mg/kg) or saline 30 minutes after pargyline (75 mg/kg) or saline treatment. 2. The administration of pargyline or m-tyrosine to saline-pretreated rats did not produce behavioural excitation. 3. The administration of m-tyrosine to pargyline-pretreated rats produced intense behavioural stimulation. 4. Pargyline administration increased brain 5-HT, DA and mTA levels. NA levels did not increase, however, until 2 hr after the drug. 5. The administration of m-tyrosine to pargyline-pretreated rats resulted in an extremely large increase in mTA levels. In contrast, NA and DA levels were significantly decreased, while 5-HT levels were unaffected except at the 2 hr time period. 6. The administration of m-tyrosine to saline-pretreated rats resulted in a large increase in mTA levels. NA and DA levels were significantly reduced, while 5-HT levels were little affected. 7. These results may be interpreted to suggest a direct mode action of mTA in the observed behavioural stimulation.