A consistent finding in exercise-associated collapse is a marked postural fall in the systolic blood pressure associated with a tachycardia. The prevalence and significance of these post-exercise (postural) changes in blood pressure among noncollapsed ultradistance athletes has not been well documented. The aim of this study was to compare pre- and post-race changes in systolic and diastolic blood pressures with changes in body weight and plasma volume and with symptoms of post-exercise hypotension, including the effects of posture, among a group of 31 runners competing in an 80-km footrace. During the race, runners developed a mean (+/- SD) weight loss of 3.5 (+/- 1.2) kg and plasma volume change of 12.8% (+/- 9.1). Asymptomatic postural hypotension defined as a fall in systolic blood pressure of greater than 20 mm Hg from the supine to the erect position without syncopal symptoms was present in two runners (7%) before the race and in 21 runners (68%) afterward. The degree of postural variation in systolic blood pressure was unrelated to changes in body weight or a fall in plasma volume. We conclude that (i) all runners were dehydrated by the race activity with a range of 1% to 7% and an average of 4.6%; (ii) asymptomatic post-exercise (postural) hypotension developed in the majority (68%) of ultramarathon runners in this study; (iii) the post-exercise hypotension is likely of multifactorial origin and is not entirely related to whole body dehydration or a reduction in plasma volume; and (iv) despite marked levels of dehydration among our sample of runners, their cardiovascular status in the supine position was not greatly compromised.