Seventeen infants who had been treated with gentamicin on the first day of life as prophylaxis for prolonged rupture of membranes or maternal fever. Twenty-seven weight-matched healthy infants were used as controls. The auditory brainstem responses were evaluated on the third day and again on the tenth day of life. Latencies of components V, intervals I-V and III-V were significantly prolonged in the gentamicin treated group when compared with the control group. This prolonged latencies reversed itself on the tenth day of life. The concentration of peak and trough levels of gentamicin were related to latency of interval III-V. These findings emphasize that a short course therapy with gentamicin in essentially healthy infants can alter the central transmission of auditory brainstem responses transiently.