Primary end-to-side anastomoses between the left upper pulmonary vein and left atrium performed in 21 puppies with continuous 5-0 polydioxanone (PDS) or polypropylene (PRO). Animals were sacrificed at 1, 7 and 28 days after operation. Stenosis occurred in one of 5 animals of the PDS group at 7 days and 28 days respectively, and one of 5 animals of the PRO group at 28 days. There was no significant difference between the two groups in terms of anastomosis patency. Histological examination showed slight tissue reaction and fibrosis surrounding the sutures in both groups. This study suggests that the absorbable monofilament suture material, in particular PDS, is useful for the repair of low pressure circulatory system and will not cause the stenosis of the anastomotic site.