In spite of abundant data about the structure of proanthocyanidins and their influence on human organism, the role of these substances in life of plants, as well as the causes of their synthesis and accumulation in seeds, are less investigated. There is a large body of data suggesting that proanthocyanidins participate in scavenging of free radicals which occur as a consequence of metabolic processes. Mentioned properties fall into the general pattern of the involvement of proanthocyanidins in oxide-reductive reactions required when elevated concentrations of oxidants appear. This could be a reason for suggesting that proanthocyanidins participate in adaptation processes of plants to diverse stresses. Taking this into account, we have initiated the study of proanthocyanidin content in grape seeds at diverse stages of berry development. For study grapes of the variety Cabernet Sauvignon (15 years old) were used. Grape samples were harvested at the following stages: 1. seed morphological completion (19 of June); 2. ripening initiation, at the moment when berry volume reaches up to 80 percent of the final volume (15 of August); 3. intensive grape ripening (19 of September). Our study showed that there are certain relations between proanthocyanidin content and biological state of the berries.