The material comprised 11 pregnant rabbits and 95 fetuses stemming from these rabbits. On the 27-th day of pregnancy laparotomy was performed and 1 microgram of thyroxin was administered to the amniotic sac of each fetus in one uterine horn. The fetuses in the second uterine horn, considered as control group, received 0.9% NaCl into their amniotic sac. On the 28-th day cesarean section was carried out, the fluid was collected, the fetuses were taken out, they were weighed and after the lapse of 45 minutes sacrificed by decapitation. The isolated lungs of the fetuses and the amniotic fluid provided the material for further studies. In the amniotic fluid, the content of lecithin and sphingomyelin was determined by thin-layer chromatography. The lungs of fetuses furnished material being used for accomplishing the following types of investigations: determination of mass weight of the wet lungs, and lungs mass after drying, biochemical examination--lecithin and sphingomyelin content in lung homogenates by thin-layer chromatography; histological examination: hematoxylin and eosin stainings by PAS method, argentation by Perdrau method, oil red, toluidine blue stainings as well as electron microscope examination. It has been disclosed that thyroxin administered into the amniotic sac accelerates the fetal lung maturation in rabbits, expressed by higher content of lecithin and by higher ratio of lecithin to sphyngomyelin in fetal lung homogenates at biochemical examinations; it exerts positive effect on the body mass of fetuses and the fetal lung mass, but it fails to influence the lung hydration degree, the sphyngomyelin content in lung homogenates as well as the content of lecithin in the amniotic fluid. The results of histological examination under light microscope have revealed that the lungs of rabbit fetuses in thyroxin group displayed in 70% morphological maturation. The fetal lungs from the control group hardly in 21.1% showed complete maturation. In electron microscopy the fetal lungs in thyroxin group demonstrated more advanced maturation than in the control group, the interalveolar septa were constructed of smaller number of cells, there was a larger number of mature pneumocytes of II type with more abundant lamellar structures, in the alveolar lumen the surfactant appeared markedly more profusely.