Both viscoelastic and plastic properties were investigated on random copolymers of methylmethacrylate (MMA) and N-methyl-glutarimide (GIM) in the range 0-76 mol%. All the measurements were performed on samples quenched from the melt in order to break free from physical aging effects. Dynamic mechanical experiments were performed at very low deformation and temperatures ranging from - 150 degrees C up to the glass transition temperature (T-g) region. Increase in GIM amount improves the thermomechanical stability of the copolymers, as revealed by the increase of both T-g and alpha relaxation temperature. In the beta relaxation region, the E " loss peak first decreases in amplitude with increasing GIM content and then broadens further and finally spreads out till the onset of the alpha peak at the largest GIM amounts. A quantitative analysis of the beta relaxation phenomena was performed by considering the loss compliance J " instead of the loss modulus E ". It turns out that in the low temperature range (-80 degrees C-0 degrees C) the mechanical damping associated with the MMA motions is stronger for MMA-GIM than for MMA-MMA linkages; in addition, the mechanical damping associated with the motions of the GIM units is very low. By contrast, in the high temperature range (30 degrees C to about 100 degrees C), the mechanical damping associated with the MMA motions drops with increasing GIM amount, whereas a significant damping coming from the GIM units is observed. These results suggest that the beta relaxation would mainly consist of MMA isolated motions at low temperature and of cooperative motions at higher temperature, involving the MMA units at GIM amounts lower or equal to 58 mol% and the GIM units at higher GIM content. The stress-strain curves were determined at low strain rate (2 X 10(-3) s(-1)) and temperatures ranging from -120 degrees C to T-g. Analysis of the plastic deformation region shows that the yield stress decreases with increasing GIM amount at low temperatures. The opposite trend shows up on the high temperature side of the beta relaxation, where strain softening peaks at intermediate GIM amounts. As a plausible explanation, the cooperative beta motions, whenever they exist, are suspected to be responsible for the decrease of both yield stress and strain softening. These conclusions agree well with those of a previous study on methylmethacrylate-co-maleimide copolymers. They are also consistent with our earlier identification of the microdeformation mechanisms involved in the stretching of methylmethacrylate-co-N-methylglutarimide thin films.