The effects of nine weeks of physical training on certain ventilatory variables and the pulmonary diffusing capacity for carbon monoxide were studied in ten university distance runners. A nonexercising control group of five medical students was also investigated. Daily calisthenics and long-distance running as well as frequent participation in competitive meets had no measurable effect on minute volume of ventilation, oxygen uptake, carbon dioxide elimination or diffusing capacity. The lack of training effect of diffusing capacity measured at a single submaximal work load confirms the results of other recent longitudinal studies. Brief review is made of previous investigations into the relationship of diffusing capacity, participation in athletics and physical training. It is pointed out that diffusing capacity is closely correlated with dimensional and functional factors of the oxygen transport system, and that training augmentation of these parameters serves primarily to extend an athlete's maximal work and endurance capabilities. The suggestion is therefore made that training effects on pulmonary diffusing capacity should be studied during maximal work loading.