Abstract Angier and Watson observed formation of spatial structures (gels) in neoprene rubber, certain SBR fractions, and butadiene-acrylonitrile rubber, under the influence of powerful shearing forces during treatment in a specially designed scroll-type masticator. This gel formation was considered to be a specific characteristic of synthetic rubbers ; no one has reported gel formation in the mastication of natural rubber (NR). In our experiments an insoluble gel was formed in NR. Weighed samples of extracted smoked sheet were masticated in an argon atmosphere containing not more than 0.05% oxygen, on specially designed rolls, with 0.12 mm gap between the rolls cooled intensive with water. The formation of an insoluble fraction (gel) at the early stages of mastication was established in repeated experiments. Its maximum content (after 10 minutes of mastication) reaches 20%, and further milling results in mechanical dispersion of the gel fraction, so that after 60 minutes of mastication the rubber is again completely soluble in the common solvents. However, the masticated rubber contains formations branched to such an extent that most of them (∼80%) are precipitated from octane solution by centrifugation at 12,000 rpm for an hour. Molecular weight determinations, based on changes in the light scattering of the solutions at an angle of 90°, showed that the molecular weights of the sol fractions of the masticated samples increase during mastication, reaching 2,500,000 after 60 minutes. The true molecular weight of the particles is apparently of major significance, as a correction for the asymmetry of the scattering particles was not applied.