Procyanidolic oligomers (PCO) are mainly used for their therapeutic effect on the vascular wall. We could show that the mechanism of this effect involves interactions with mesenchymal cells and extracellular matrix (ECM). Recently we demonstrated in vitro that they also act on cornea, a tissue rich in ECM. For instance they stimulate the corneal biosynthesis of type VI collagen and proteoglycans. A potent antiprotease effect also could be demonstrated on corneas. In our present work we examined in vivo action of PCO on corneas. A group of rabbits received during 1 week on their right eye a treatment by procinaidolic oligomers, the left eyes were kept as controls. After 1 week both eyes underwent excimer laser photobalation. Another group of rabbits also received the PCO treatment on the right eye, not on the left, but was not treated by photoablation. One week after the surgical intervention corneas were collected and biochemical and morphological observations were carried out. Ocular administration of PCO was well tolerated and no toxic or inflammatory side-effects could be seen. In the control group photoablation was followed by a decrease of the content of corneas in type I collagen and a strong increase in type III collagen. On the corneas treated by PCO these alterations of the composition were not observed. These results indicate that PCO treatment before excimer laser photoablation maintains within normal limits the biochemical composition of the operated corneas.