ABSTRACT: Under certain conditions of deep drilling with PDC bits, violent stick-slip torsional vibrations take place in the drilling structure. In the classical approach used to analyze this problem, it is assumed that stick-slip events arc triggered by a bit-rock interaction law, characterized by a decrease of the torque with angular velocity (which is formally equivalent to a decreasing apparent friction coefficient with angular velocity). Based on laboratory data, we argue that this relationship is not an intrinsic characteristic of the bit-rock interaction. We then elaborate on the coupling between vertical and torsional vibrations at the bit as a possible primary cause of stick-slip. Preliminary results obtained with this model indicate that such coupling is sufficient to generate stick-slip without the need to assume a decreasing friction coefficient.