The intrathoracic pressure was determined by direct intrapleural cannulation in 17 clinically normal horses and 14 horses with chronic obstructive pulmonary disease (COPD). There were significant differences between the normal and COPD horses with regard to max. Ppl and max. Ppl. The mean values for minimum Ppl of the 2 groups of subjects were not significantly different. The results were discussed in relation to those of other workers.