In order to elucidate the relationship between the hearing loss and the diminished thyroid function, experiments were designed to study the effect of hypothyroidism on the auditory brain stem response in 17 adult myxedematous rats, which were induced by thyroidectomy or daily oral administration of propylthiouracil(PTU) 1.5-2 mg for 8 wks. The results were as follows ; 1) Normal ABR showed 5 peaks within 5 msec. Wave I : 0.76±0.06 msec, Wave II : 2.12±0.11 msec, Wave III : 3.13±0.10 msec, Wave IV : 3.76±0.09 msec, Wave V :4.82±0.12 msec. 2) Normal serum T4 level was 4.19±0.84 μgm % and that of the thyroidectomized or PTU intake groups was nearly below 1.0 μgm%. 3) Normal wave I-V interpeak latency was 4.06±0.12 msec. After oral administration of PTU, latency was prolonged for 0.37±0.6 msec(p<0.001). 4) Wave I-V interpeak latency of the thyroidectomized group was prolonged for 0.31±0.11 msec(p<0.001). 5) There was no significant change on ABR patterns than control group in 6 wks after stopping the administration of PTU. 6) There was no statistical difference in the wave I latency between the normal & hypothyroidism groups. 7) This finding may indicate a possible site of lesion in the brain stem in hypothyroid rats.