The postnatal maturation of the auditory brainstem response(ABR) has been studied in several species, including human, cat, rat and gerbil. However, only one paper was published regarding the development of ABR in the guinea pig. The purpose of this study is to clarify the pattern of ABR maturation with aging in the guinea pig. A total of 39 albino guinea pigs were used. The latencies, amplitudes and forms of each waves were observed on the 1st, 4th, 7th, 14th, 21st, 28th and 90th day after birth, respectively. The ABRs were picked up at the vertex using subcutaneous electrodes and DANAC 7E ERA System(Japan). Animals were anesthetized with secobarbital(20-30mg/kg) and body temperature was maintained at 37° with water-heating pad. The filtered click stimuli were presented to the animal with 90dB SPL of 1kHz, 2kHz, 4kHz, 8kHz at 8.8 times per second of repetition rate. A total of 100 stimuli were presented and averaged. The threshoeds of ABRs in the animals of the first day after birth were not different with those in the animals of 90th day after birth. The rapid decrease of latencies of each waves were noticed during the first 2 weeks of life and then the gradual stage was observed, reaching the adult-level at 3 to 4 week after birth. In addition, with aging, amplitudes of wave I, II, III were decreased and in contrast, those of wave IV, V were increased. Our results suggest that the development of ABRs are completed at 3 to 4 week after birth and occurs in distinct 2 phases, the early rapid and late gradual stages.