In 1973, the Institute of Environmental Stress of the University of California-Santa Barbara, under the direction of Steven M. Horvath, began a series of field and laboratory studies of marathon runners during competition. As one of Horvath's graduate students, many of these studies became part of my doctoral dissertation. The rationale for studying runners under race conditions was based on my belief as a marathoner that runners would push themselves much harder while competing than under simulated conditions in the laboratory. Horvath's ready support of the studies likely had its roots in his graduate training at the Harvard Fatigue Laboratory, a laboratory well known for its field studies of individuals working in extreme environments. This report describes the studies of 1973-1975, focusing on how the measurements were made and detailing the learning experiences of a new graduate student. In 1973, blood chemistry and fluid shifts were studied in six runners before and for 3 days after a race. This was the first modern study to systematically examine the recovery process. In 1974, oxygen consumption was measured every 3 mi. in two runners during the race. In 1975, rectal temperature and five skin temperatures were evaluated in the same two runners every 1.4 mi. of the race. The latter two studies were the first to make such measurements under race conditions. The Institute of Environmental Stress marathon studies demonstrated the possibility of making measurements during competition without disrupting performance, enhanced our understanding of human exercise capacity under competitive conditions, and provided new insight into the postrace recovery process.