Transient renal responses to close arterial instantaneous injections of NaH2PO4 and creatinine (ca. 500 µmoles of each) were studied in dogs under conditions of high urine flow with ‘normal’ (0.5–1 mm/l.), and with high (3–5 mm/l.) levels of serum phosphate. When serum phosphate was at normal levels (no exogenously administered phosphate), the increment of phosphate excretion per unit injected per one circulation of injected substance was equal to or less than that of creatinine. When serum phosphate was elevated (by continuous infusion of NaH2PO4 for 1 hr. preceding injection), the increment of phosphate excretion per unit injected per one circulation exceeded that of creatinine. Such findings suggested the existence of a tubular secretory mechanism for phosphate. Dependency of the postulated mechanism upon cellular reserves and/or plasma concentration of phosphate is discussed.