Abstract: Method for automatically adapting and configuring the speed of a terminal adapter (30) to the rate 56 Kbps or 64 Kbps which is used by a calling adapter (20). After sending the CONNECT message via the ISDN NETWORK to the calling terminal adapter (20) in accordance with CCITT Q.931 Recommendations, the called TA (30) is initialized to a rate of 64 Kbps and then continuously transmits (204) an alignment pattern ALL.sub.-- ONES while starting a first timing process (T1). This first timing process will cause the called TA to switch to a 56 Kbps speed if the 64 Kbps validation process does not succeed. The method then involves the step of checking (206) the reception of a ALL.sub.-- ZEROS pattern coming from said calling adapter (20) before the end of said first predetermined period (T1). If this case, a 64 Kbps validation process will be performed which comprises the checking whether a ALL.sub.-- ONES pattern is received within a second period (T2), in which case the 64 Kbps rate configuration will be validated.