Many details concerning CSF production at the choroid plexus remain unsettled, and the control mechanisms on the production are also not yet clarified. In the present study, the association of serotonin in the dynamics of the CSF production was studied. The authors utilized the ventriculocisternal perfusion method developed by Pappenheimer and an automatic continuous measurement technique which was designed by the authors. The experimental animals comprised 48 mongrel dogs weighing 7.0 to 12.0 kg. The animals were inactivated by intravenous injection of Dialferin after intratracheal intubation for generalized anesthesia with N2O, O2 and Halothane. Within about 2-2.5 hours after starting perfusion of the Blue Dextran-Elliott-B perfusion fluid, the optical density reached a plateau, and the CSF production in the control dogs could be calculated. Once the data for the control had been obtained, intravenous administration of 5-hydroxytryptophan (5-HTP), noradrenaline (NA) and acetazolamide (Diamox) was commenced. Administration of various amount of 5-HTP, a precursor of serotonin, reduced the CSF production, that was, after administration of 10 mg/kg, 20 mg/kg, and 30 mg/kg 5-HTP the CSF production reduced by 37.0 +/- 4.4%, 39.9 +/- 3.1%, and 35.2 +/- 5.5% (mean +/- SD), respectively, and no correlation with dose was observed. After administration of 5-HTP, the blood pressure showed unsteady state for about 10 minutes, showing an increase in pulse pressure. It then declined gradually. All these phenomena were within the range of autoregulation.(ABSTRACT TRUNCATED AT 250 WORDS)