Studies were made of the physiologic changes in the lung of young growing pigs intratracheally inoculated at 5.5 to 6 weeks of age with Mycoplasma hyopneumoniae. Hematologic changes also were studied. Studies were performed at the 4th, 5th, and 6th weeks after inoculations were done. The inoculated pigs had a marked decrease in body weight gain. There were no significant changes in the heart rate, respiratory rate, and rectal temperature, even though at necropsy the apical, cardiac, and diaphragmatic lobes of the lungs of inoculated pigs had scattered, but well-demarcated, pneumonic lesions. The lungs were edematous and increased in weight, compared with those of control (noninoculated) pigs. There were several changes in respiratory functions in the inoculated animals, including significant increases in esophageal pressure negativity, expiratory flow rate, tidal and minute volumes, and work of breathing and significantly less oxygen consumption. Inspiratory flow rate and airway resistance were markedly increased. The elasticity of infected lungs was reduced, as shown by notable decreases of dynamic lung compliance. According to blood gas analyses, acid-base balance had changed in the inoculated pigs. Arterial blood pH, arterial partial pressures of O2 and CO2, and arterial concentration of HCO3- decreased. To some degree respiratory acidosis indicated by a significant decrease of base excess was evident in the blood of inoculated pigs. As a result, inoculated animals required hyperventilation (compensatory respiratory alkalosis) to maintain the pH of arterial blood near a physiologically normal value. The pigs with mycoplasmal pneumonia showed no drastic hematologic changes.(ABSTRACT TRUNCATED AT 250 WORDS)