This study investigated the question of hemoconcentration-hemodilution and subsequent vascular fluid shifts evidenced by marathon runners. Blood samples were taken from runners before and after the New York City Marathon of 1978 and the Fiesta Bowl Marathon of the same year. Participants were of both sexes. Tables accompanying this report present information on the characteristics of the subject groups, hematological changes elicited by marathon running, serum electrolytes and osmolality following a marathon, and percentage changes in volumes of blood, plasma, and red cells following a marathon race. (JD)