The ductus arteriosus was studied in 138 dogs (80 female and 58 male) aged from 6 h to 21 days. This experiment led us to conclude that the anatomical closure of the d. arteriosus starts within 96 h and is obliterated after 6 days in 100% of the animals studied. We have also evidenced that the closure starts in the medial part of the d. arteriosus and progresses to the pulmonary artery and aorta.