The study of cold mastication of hydrogenated nitrile rubbers containing from 34 to 43 wt.-% of acrylonitrile showed that these rubbers undergo significant degradation due to action of mechanical shearing forces. The prolongated mastication time results in limited molecular weight of rubber M-n,M-lim = 58,5 to 66,5 kg/mol. Both the degradation rate and the resulting degradation level or degradation degree determined by the decrease of the average molecular weight are almost not affected by the concentration of residual double bonds or bound acrylonitrile content in HNBR as well. The decrease of molecular weight of all rubbers studied can be described by the same first order rate constant k(1) = 4,80 (+/- 0,22) 10(-4) s(-1). The cold mastication of HNBR is accompanied by the decrease both of Mooney viscosity and by the narrowing of the molecular weight distribution, From GPC studies it follows that the degradation of rubber chains occuring due to cold mastication of hydrogenated nitrile rubber is not a statistical process.