Collagen, elastin and structural glycoprotein content of the lungs of 38 fetal and neonatal foals, 8 of which were showing dysmaturity or convulsive syndrome, were measured by standard biochemical means. Glycoprotein content showed little or no change between 100 and 340 days of gestation; elastin remained constant from 100 to about 260 days when there was an exponential increase up to the time of birth, while collagen content rose linearly from 100 days to birth. In dysmature animals there was significantly less collagen in the lungs at birth but the difference in elastin content between the two groups was not statistically significant. There was no clear distinction between glycoprotein content of the dysmature and normal animals. Histological studies indicated that the lung of the fetal horse matures early and in the last third of pregnancy the adult pattern of blood vessels and trabeculae had already appeared. The reduced collagen content in the dysmature foals might be associated with anomalies of collagen synthesis and cross-linking. These could weaken the structure of the lungs and blood vessels and lead to haemorrhages, especially those in the central nervous system, which are a feature of the dysmature syndrome.