To compare plasma NT-proANP, a stable and biologically inactive N-terminal portion of ANP prohormone, with the known plasma ANP response to increased right atrial pressure a Swan-Ganz catheter was inserted into the right atrium of five normal healthy male volunteers. The elevation of right atrial pressure was produced by a head-down tilt after a hypertonic saline infusion. Blood samples were drawn from the lumen of the right atrium. After 5 min of starting the tilt the right atrial pressure had increased from 7.0 +/- 1.0 to 11.6 +/- 0.9 mmHg (P < 0.05) and then began to normalize in spite of the constant tile. Atrial plasma ANP increased in relation to the pressure increase and peaked at 15 min after the start of the tilt. The change was from 27.9 +/- 6.5 to 53.9 +/- 9.7 pmol L-1 (P < 0.05). Atrial plasma NT-proANP increased significantly from 357 +/- 91.2 to 529.1 +/- 116.0 pmol L-1 (P < 0.05) at 10 min and remained high throughout the experiment. The molar ratio of NT-proANP to ANP varied in atrial plasma from 9.5 +/- 1.2 to 13.9 +/- 2.7 showing that the plasma clearance of ANP from plasma was much higher than that of NT-proANP.