A circuit for generating an internal source voltage signal responsive to an external source voltage signal in a semiconductor memory device prevents malfunction and extends the lifetime of the device by clamping the internal source signal if the device is in a normal operating mode when the external source signal is in a stress operating range. When the device is placed in a test mode, the circuit allows the internal source signal to increase in proportion to the level of the external source signal when the external source signal is in a stress operating range. The circuit includes in internal source voltage generator, which always clamps the internal source signal when the external source signal is in a normal operating range, and a pull-up unit which is activated in response to a control signal. The control signal is enabled when the device is placed in a test mode by combining external timing signals.