The present study was designed to investigate the effect of administration of adrenomedullin (ADM) into subfornical organ (SFO) on renal tubular Na(+),K(+)-ATPase activity in rats. Rats under anesthesia were injected with ADM 0.1 mL (20 ng/mL) via an implanted cannula into SFO (n=6). Plasma ADM and serum endogenous digitalis-like factor (EDLF) levels were assayed with radioimmunoassay, and urine samples were collected via a canoula intubated in bladder. Urinary sodium concentration was assayed with flame spectrophotometry. Single proximal renal tubule segments were obtained by hand under stereomicroscope and its Na(+),K(+)-ATPase activity was measured by liquid scintillation counting. In addition, single proximal renal tubule segments from normal rats (n=6) were incubated with serum from animals administered with ADM into SFO, and then the Na(+),K(+)-ATPase activity was determined. The results showed that both urinary volume and sodium excretion amounted to the peak value at 30 min after ADM administration, and sustained a significant high level at 60 min (P<0.01). At 30 min after ADM administration, there was a significant increase in serum EDLF and a decrease in Na(+),K(+)-ATPase activity of proximal tubule (P<0.01, respectively), but not in plasma ADM level. Na(+),K(+)-ATPase activity was decreased significantly in single proximal renal tubule segments from normal rats incubated with serum from rats administered with ADM into SFO (P<0.01). These results suggest that the diuretic and natriuretic responses following administration of ADM into SFO are associated with the inhibition of renal tubule Na(+),K(+)-ATPase activity. The inhibition of renal tubule Na(+),K(+)-ATPase activity is related to the increase in the serum level of EDLF.