Eleven patients with pulmonary sarcoidosis of type II or III were investigated with regard to regional distribution of ventilation and perfusion (V/Q), alveolar gas exchange and diffusion limit at rest and during exercise. Lung volumes were 50-65% of normal values. Flow-volume curves indicated obstructive changes. The transfer factor was 75% (range 16-120%) of predicted. Perfusion scintigraphy showed marked defects in 7 out of 11 patients. Radiospirometry showed matching ventilation and perfusion defects and washout of xenon was prolonged. There was a venous admixture at rest of 9%. Arterial oxygen tension (Pao2) averaged 9.7 kPa. V/Q analyses indicated the presence of a small shunt (1%), regions with low V/Q in 4 out of 11 patients, regions with high V/Q in 5 out of 11 patients and increased wasted ventilation. At rest measured Pao2 was lower (0.6 kPa) than predicted from the V/Q distribution. During mild supine exercise causing significant dyspnoea, pulmonary vascular resistance rose to abnormal values, 5.2 mmHg.l.-1 min-1.m2 BSA. The venous admixture decreased to 5.4%. The shunt was unchanged, as was the perfusion of regions with low V/Q. The regions with abnormally high V/Q disappeared. Measured Pao2 decreased to 9.1 kPa, while calculated Pao2 remained unchanged. Thus the P(A-a)o2 at rest (4.2 kPa) was 70% caused by shunt and V/Q mismatch. During exercise alveolar-arterial pressure difference for oxygen measured P(A-a)o2 rose further to 5.1 kPa, while calculated P(A-a)o2 remained unchanged and was only 50% caused by shunt and V/Q disturbances. The difference between calculated and measured Pao2 indicated significant diffusion limitation both at rest and during mild exercise.