Acute experiments on mice and rats were made to determine the intraperitoneal lethal doses of the polyribonucleotide complexes poly (I).poly (C) and poly (G).poly (C) manufactured in this country. Changes in the function of cardiovascular, nervous, thermoregulatory and motor systems seen after injection of both complexes were shown to have some features in common but to differ in the rate and severity of poisoning. It was disclosed that the test polyribonucleotide complexes might be attributed to a group of substances that manifest the medium degree of cumulation.