The object of the present study was to observe the effect of 7%» CO2 breathing on pulmonary vasculature and on the right of ventricle. Two batches of twelve rats were giv-en interrupted exposures for 6 h daily during 3 or 6 months while a third batch of 12 was given continuous exposure for 288 h. 36 rats were kept as controls. Medial hypertrophy was assessed by calculating the percentage of medial thickness. Changes in right ventricular muscle were assessed by calculating the ratio of the weight of left ventricle plus septum to right ventricle. There was a significant increase in medial thickness of muscular pulmonary arteries – the longer the period of exposure, the greater the thickness – the mean percentage medial thickness, going up from 4.06 in controls, to 15.3 in animals exposed for 6 months. Significant right ventricular hypertrophy was observed in rats exposed for 3 and 6 months. It is felt that frequently repeated constriction of muscular pulmonary arteries produced medial hypertrophy and increased pulmonary vascular resistance resulting in right ventricular hypertrophy