Mean fat cell diameter and lipolytic activities of adipose tissue were determined in five ex-obese runners (EOR) who had experienced a mean weight loss of 39.5 kg during the course of a long-distance running program. At the time of investigation, their mean weekly running distance was 95 km in which no diet manipulation was needed to maintain their body weight. Body fat was estimated to be 14.3 percent of their body weight. Despite the high amount of exercise, subjects were not able to reduce body weight any further. Their values were compared with those obtained in the two following groups (1): six elite long-distance runners (ER) having a lower percent body fat (percent fat = 9.5); (2) five sedentary controls (SC) who were paired for adiposity with EOR (percent fat = 14.4). All subjects were submitted to a biopsy of subcutaneous fat in the suprailiac region, in which mean fat cell diameter was 57.5, 74.5, and 86 microns in EOR, ER and SC respectively. Basal and epinephrine-stimulated lipolysis were similar in EOR and SC, while ER exhibited higher values. These results indicate that exercise-training can produce important weight losses when subjects are capable of tolerating high levels of energy expenditure for extended periods of time. Moreover, the reduced fat cell diameter for a higher adiposity in EOR as compared to ER suggests that resistance to further fat loss may occur when the fat cell size is markedly reduced. No deficit in maximal epinephrine stimulated lipolysis of isolated fat cells was found in EOR in comparison to SC subjects, but they failed to adapt like lean ER subjects.