The composition of polyuridilic acid hydrolisis products with mol. weight 400.000--600.000 by endoribonuclease from cobra venom was studied. The molecular weight distribution of products formed at initial stages of hydrolysis was according to the regularities of accidental cleavage of phosphodiether linkages. The sufficient non-statistic regularities due to the high-molecular polynucleotide hydrolysis for the most part during further hydrolysis were observed out of the way of accumulation of oligonucleotides with a polymerity degree of about 15--20. As shown from the oligocytidilates hydrolysis, the velocity of process increased sufficiently with increasing substrate polymerity degree. The results obtained were used for preparation of polyuridilic fractions with a narrow range of molecular weight distribution.