The present study was performed to assess the reliability of lower body negative pressure (LBNP) as a test of orthostatic tolerance. The need for this assessment arose from the prior observation in this laboratory that some subjects show wide day-to-day variation in heart rate responses to LBNP. The extent of these variations was so great as to a raise a serious question as to the value of LBNP as a measure of study-induced alterations (e.g. those produced by bedrest or weightlessness) in orthostatic tolerance. Five healthy volunteers were subjected to a series of tests, consisting of 70 degrees tilt, LBNP, and passive standing. on three occasions preceding and three occasions following a 2 week period of bedrest. Study results show that it is possible to subdivide the volunteers into subgroups which show either great or little day-to-day variability in any of the three tests. All three tests reaveled bedrest-induced alterations in orthostatic tolerande quite adequately. Of the three tests studied, LBNP most frequently resulted in the largest test-induced heart rate alterations, followed by quiet standing and, finally 70 degrees tilt