A research report submitted to the Faculty of Medicine, University of the Witwatersrand, in partial fulfilment of the requirements for the degree of Master of Science in Medicine in Applied Physiology. Johannesburg, 1997. / A post-exercise reduction in blood pressure (BP) may be the primary reason that athletes suffer from exerclse-assoclated collapse (EAC) at the end ot ultra-endurance running ever.s. Plasma volume decreases, possibly caused by dehydration, may be the cause of the decrease til blood pressure, In order to determine whether there is a correlation between plasma volume changes and the post-exercise BP drop, this study evaluated alterations in pre- and post-race blood pressures and changes in blood and plasma volumes, It found that compared to resting values, systolic, dlastollc and mean arterial blood pressures (mmHg) fell significantly from 119 ± 4, mean ± standard deviation, 74 ± 8, and 88 ± 5 respectively to '106 ± 14, 62 ± 12 and 77 ± 10 (ps 0,05), whereas pulse pressure failed to change, Compared to pre-race values, plasma and blood volume were found not to have changed significantly, During the race plasma urea (U) and creatinine (C) concentrations increased significantly, whereas body mass and body mass index both fell significantly. Haernatocrlt, haemoglobin, mean cell volume, red blood cell number, mean cell haemoglobin concentration, the mean cell haemoglobin, plasma sodium, potassium, chloride and protein concentrations, the U:C ratio and osmolality remained constant. There were no significClnt correlations between changes in plasma or blood volume and changes in blood pressure, These data support the Idea that a post-race decrease in blood pressure does not result primarily from an intravascular fluid loss, It is likely therefore that athletes who collapse at the end of ultraendurance races due to EAC do so as a result of 'post-exercise hypotension' secondary to venous pooling, and not as a result of a reduction in plasma volume, / MT2017