The therapeutic effectiveness of inversion traction has only recently come under investigation. The purpose of this study was to use noninvasive methods to determine traction, cardiovascular, and perceived effects of inversion. The Gravity Gym(R) and Gravity Boot(R) inversion systems were compared. Treatment order was randomized and 18 normal subjects between the ages of 22 and 31 were suspended in each inversion device for 7 minutes. A computerized method was used for assessing posture. Three-dimensional coordinates were determined for spinous processes C7 through L5 relative to the midpoint of a line drawn between posterior superior iliac spines (S2). Heart rate was palpated and blood pressure was recorded using a standard sphygmomanometer. Pre-, initial, late, and postinversion measurements were taken. Questionnaires administered following each inverted posture assessed levels of discomfort. One-way analysis of variance was conducted to test the significance of results (p < 0.05). Measurements for both systems revealed an increase in L5-S2 distance, a decrease in C7-T12 distance, and a decrease in depth of the thoracolumbar curves. Segments C7-S2, L 1 -S2, L 1 -L2, L3-L4, and L4-L5 were increased in length by the Gravity Gym but not the Gravity Boots. Heart rate decreased and blood pressure increased an average of 20 mm Hg for both systems as subjects were inverted. This pattern was observed throughout the inversion period. Upon return to stand, heart rate increased for both systems while blood pressure decreased only for the Gravity Boot system. Subjects indicated that the Gravity Gym was the more tolerable inversion system. lnversion can be an effective means of spinal traction but due to elevated blood pressure, inversion may be contraindicated for some patients.J Orthop Sports Phys Ther 1986;7(5):254-260.