Since existing data suggest decreased pulmonary gas exchange at rest when hyperlipemic, the effect of alimentary hyperlipemia on steady-state pulmonary diffusing capacity for carbon monoxide (DLco) at rest and exercise was investigated. Twelve men were measured on two mornings after a 12-hr fast. One trial was performed with subjects in the fasting state, and the other three hours after ingestion of one gm fat per kg body weight. At each trial, venous blood was drawn at 0800 and again three hours later just before resting DLco measurement. The DLco was subsequently determined at a work intensity on the cycle ergometer of 49 watts, and then at an increased intensity sufficient to raise the heart rate to 150 bpm. Serum was examined spectrophotometrically to assess degree of lipemia as reflected by optical density. The DLco and serum optical density data were analyzed by ANOVA. Lipemia was significantly increased three hours after fat ingestion (p < 0.05). Although mean DLco, in ml/min/mmHg was lower following fat ingestion at both intensities of exercise (49W = 37.1 fasted; 34.5 fed: 150 bpm = 49.8 fasted; 44.6 fed), alimentary hyperlipemia did not significantly affect resting or exercise DLco.