We used intracerebral dialysis to monitor extracellular levels of dopamine and its major metabolites dihydroxyphenylacetic acid (DOPAC) and homovanillic acid (HVA) in rat striatum. Levels of these compounds were determined after intraperitoneal administration of phenylalanine (200, 500 and 1000 mg/kg). A dose of 200 mg/kg phenylalanine increased basal dopamine release by 59%, peaking at 75 min. There was no change in basal dopamine release after the 500 mg dose, whereas the 1000 mg/kg dose significantly reduced (26%) dopamine release. No significant changes were observed in the concentrations of DOPAC and HVA with any of the treatments, indicating that changes in brain phenylalanine and tyrosine levels may selectively affect production of the dopamine molecules that are preferentially released into synapses.