The cerebrospinal fluid (CSF) system of two amphibians, Rana catesbeiana and Rana pipiens has been perfused from the lateral ventricle to a site in the subarachnoid space close to the roof of the fourth ventricle. The CSF production rate (Vf) was measured by the dilution method using a perfusion fluid containing [14C]dextran in artificial CSF. For Rana catesbeiana Vf was 1.43 microliter min-1 or 0.2 microliter min-1 mg choroid plexus-1 and for Rana pipiens Vf was 0.2 microliter min-1 or 0.1 microliter min-1 mg choroid plexus-1. These control values declined by 35% and 14% respectively in the second half of the experimental period. Ouabain (5 X 10(-5) M) in the perfusion fluid for the second half of the experiment resulted in a much greater reduction in Vf, viz. 65% for Rana catesbeiana and 71% for Rana pipiens. Acetazolamide (1 X 10(-4) M) in the perfusion fluid for the second half of the experiment gave no change in Vf values when compared with control 74.5 +/- 4.8 microliters.