Some profoundly deafened patients, who cannot be helped by sound amplification, claim to perceive auditory sensations when alternating currents up to 100 Hz are passed through electrodes applied to the skin of the external ear canal. A portable speech processor has been developed which supplies this current. The signal is a balanced square wave, the frequency of which is proportional to the first formant frequency. The amplitude is proportional to the intensity of voiced sounds. In order to fit the narrow frequency and dynamic range of the electrical stimulus, the speech processor produces a downward frequency transposition and strong limitation of the dynamic range. The device has been tested for (1) discrimination of environmental sounds; (2) question/statement discrimination; (3) identification of vowels and consonants in vowel/consonant/vowel context; (4) lip-reading with and without the prosthesis.