Eleven women long distance runners (mean age of 32.4 years) of national and international caliber were assessed for body composition and cardiovascular en-. durance. The mean relative body fat was 15.2% which is approximately 50% of the value normally found for females of this age, but more than 100% above the values found for men endurance runners of a similar age. Vo2 max averaged 59.1 ml/kg.min which is considerably higher than normal values for either girls or women. This is 16% lower than the value reported for men marathoners of national caliber, but this difference is reduced to 7.8% when expressed relative to lean body weight. When the three best runners were compared with a small group of elite male runners of similar age the results suggest that the large differences observed between normal males and females in previous studies at all ages beyond the age of 12 years are at least partially social-culturally determined as opposed to being strictly of biological origin.