Maturation of the peripheral auditory system was monitored with the brainstem response audiometry (BSRA) technique in L-thyroxine (T4)-injected rats. Neonatal hyperthyroidism (NH) was induced by daily injections of T4 from birth to 20 days. Following an initial period of precocious onset and accelerated maturation of click-evoked cochlear activity, specific tone stimulation triggered an altered evoked activity as soon as 16 days after birth with no recovery on the last recording session at 120 days, when the animals are considered adult. These results thus indicate that NH could rapidly lead to a permanent abnormality of peripheral auditory system evoked potentials.