Plasma antidiuretic hormone (ADH) levels during head-up tilt were measured in quadriplegic patients and normal healthy subjects. In normal subjects, neither rapid tilt (from horizontal to 70° head-up in 20 seconds and remaining head-up for 5 minutes) nor gradual tilt (from horizontal to 70° head-up over 15 minutes) significantly changed the plasma ADH level. In contrast, rapid tilt elevated plasma ADH levels approximately fourfold in quadriplegic subjects. A less dramatic elevation in plasma ADH levels followed gradual tilt in quadriplegic subjects. These data, along with the possibility that the pressor potency of ADH is enhanced in quadriplegic subjects, suggest that ADH is involved in the maintenance of blood pressure in quadriplegics in a head-up position.