Gamma-hydroxybutyric acid (GHBA) in doses that increased the striatal dopamine (DA) content of rat brain failed to increase the affinity of striatal tyrosine hydroxylase (TH) for its pterdine cofactor or to change the sensitivity of the enzyme to the inhibition by DA. Haloperidol (1 mg/kg) decreased the apparent K mof striatal TH for the pteridine cofactor. However, when GHBA was injected before haloperidol it prevented the decrease in the apparent K mof TH, in a dose related manner. In vitro GHBA (10−4 M) neither changed the stimulation of the striatal adenylyl cyclase by DA nor its inhibition by haloperidol. These results suggest that in striatal dopaminergic terminals the K mof TH for the pteridine cofactor is regulated by an molecular mechanism which requires that the impulse flow in the DA neurons is unimpaired.