The ability of type two superconductors to carry large critical currents, in the presence of high magnetic fields, is related to the pinning force acting on the flux lines. In the new high temperature superconductors, the pinning barrier energy is relatively low, and flux motion effects are unusually large. The knowledge of the pinning energy is very important in order to understand the transport properties of these materials. Recently, it has been shown that the broadening of the resistive transition of single crystals in the presence of magnetic fields can be related to thermally activated flux motion (flux creep). This paper reports on an investigation of the temperature dependence of the resistance of Bi/sub 2/Sr/sub 2/CaCu/sub 2/O/sub y/ and YBa/sub 2/Cu/sub 3/O/sub y/ single crystals in the presence of an external field of up to 6 Tesla with the magnetic field directed along the c axis. The measurements were performed with a d.c. bias by a four leads configuration. The data are discussed with reference to the thermally activated flux motion and with the temperature dependence of the activation energy taken into account.