Plasma atrial natriuretic peptide (ANP) was measured with radioimmunoassay in 39 patients with acute stroke. The mean level was 144.3 +/- 8.7 pg/ml; this was significantly higher in comparison with those in normal controls (55.8 +/- 4.7 pg/ml, n = 20) and hypertension patients (87.7 +/- 8.0 pg/ml, n = 15). Serial measurement of ANP level in 6 patients showed that it was elevated on the first day of the stroke, reached to its peak on the second or third day and returned to normal after two weeks. Five patients had hyponatremia following the attack of stroke and the plasma ANP level was further elevated. It is suggested that elevated plasma ANP level might be a pathogenetic factor of the accompanying hyponatremia in stroke.