We calculated the rates of lateral and third-ventricular (LTV) cerebrospinal fluid (CSF) formation in three patients undergoing metrizamide computed tomography (CT) ventriculography with reference to Ct estimates of ventricular volume and the time course of LTV metrizamide washout (elimination). Elimination rate constants for metrizamide were derived from measurements of LTV metrizamide concentration on serial postventriculography CT scans, and LTV volumes were estimated by partial volume analysis. CSF formation in the combined lateral ventricles averaged 0.14 ml per minute, and third-ventricular CSF formation averaged 0.014 ml per minute. Thus, the third ventricle accounted for approximately 10% of total LTV CSF production, which accords with relative weights of choroid plexus tissue in the lateral and third ventricles.