The medial thickness of the small pulmonary arteries was measured histometrically on sixty one autopsied cases, ranging from still born infants to five year old children. The studied cases were classified in to four groups. (1) Infants died of v arious diseases except of pulmonary disorders. (2) Wilson-Mikity Syndrome. (3) Massive aspiration syndrome of mature infants and hyaline menbrane disease of premature infants. (4) Congenital heart diseases.<BR>The results were summarized as follows.<BR>(1) Infants died of various disease s except of pulmonary disorders. a) The thickness of media at 100μ radius wide pulmo nary arteries were 1.47μ in embryos,9μ in neonates,7.8μ in six month infant,7it in one year old infant,6μ in two years old child,5.5p in three years old child, and 5μ in five years old child. The media of the small pulmonary arteries is relatively thick in embryonic period and becomes rapidly thinner after birth, especially remarkable by six months. After 6 months and up to 5 years decrease of the thickness is slow and it becomes the same as adult after 5 years. b) The media thickness of the small pulmonary arteries is rather thin in prem ature infants without. pulmonary disease. The thickness at 100μ radius wide arteries was ranging from 8.5μ to 3.5μ.<BR>( 2 ) Wilson-Mikity S yndrome Thickness of the me dia at 10μ wide arteries showed the higest value,20μ in average. This thickest media may be caused by prolonged anoxia and resulted in pulmonary hypertension which is obviously effective to develop cor pulmonale in this syndrome. This observation is an important clue to study the causes of W-M Syndrome.<BR>(3) Massive aspiration syndrome of mature infants and h yaline menbrame disease of premature infants. In both mature and premature infants, thickness of the media at 100μ wide arteries were 11.7μ and 12.4μ in average, respectively. They were thicker than those of infants without pulmonary lesions. The thinning of the muscular media after birth would be insufficient in these pulmonary diseases.<BR>(4) Congenital Heart Diseases Thickness of the med ia at 10μ wide arteries was 13μ in average which is 2.4 times thicker than those of infants without pulmonary diseases. Though the variation was wide, it was likely that either anoxia or pulmonary hypertention causes a pulmonary vasoconstriction in some cases of congenital heart diseases then cases as in massive aspiration syndrome and hyaline menbrane disease.