Recently, dantrolene has been applied to the treatment of neuroleptic malignant syndrome. In some treated patients, dramatic changes in the electroencephalogram and in the levels of monoamine metabolites in cerebrospinal fluid were observed. Nonetheless, little is known about the effect of dantrolene on the central nervous system. In this study, I investigated the effects of dantrolene on rat brain tryptophan hydroxylase (TrpOHase) activity. Intraperitoneal administration of dantrolene (20mg/kg) increased the TrpOHase activity to approximately 130% of the control level. An increase in TrpOHase activity was found in a synaptosomal fraction. Activation of TrpOHase by dantrolene was observed in a dose dependent manner in vitro, and the highest activity was obtained with 6.0 μM dantrolene (approximately 125% of the control level). Preincubation with dantrolene had no remarkable effect on TrpOHase activity. The effect of dantrolene on TrpOHase was manifested kinetically as a decrease in the apparent Km of the enzyme for cofactors (BH(4) and 6MPH(4)).Dantrolene had no effect on TrpOHase partially purified by affinity chromatography. These findings suggest that dantrolene influences the central nervous system and alters TrpOHase activity.