The use of diagnostic ultrasound raises questions regarding its bioeffects upon fetal auditory function. This study was designed to determine whether ultrasound produces changes in the auditory brain stem response in fetal lambs. Near-term fetal lambs were chronically instrumented with electroencephalogram electrodes at the vertex and the right mastoid region. An earphone was secured in the fetal right external ear canal for auditory stimulation. An ATL mechanical sector scanner was used for all ultrasound studies at a spatial peak, temporal average intensity of 15.5 mW/cm2. Auditory brain stem responses were recorded at five and 15 minutes during pulsed-wave ultrasound exposure of the cranium and then 30 minutes after cessation of insonation. Sham experiments (without insonation) were then performed in a similar time sequence to rule out the effects of habituation. There was a consistent decrease in the mean amplitude and a consistent increase in the mean latency of all five wave deflections of the auditory brain stem response during insonation. These changes persisted for 30 minutes following cessation of insonation, after which auditory brain stem response amplitude and latency values returned toward control values. No changes in auditory brain stem response amplitude or latency were observed during sham experiments. We conclude that direct ultrasound exposure of the fetal cranium through the maternal abdominal wall may temporarily influence nerve conduction along central nervous system axonal pathways. This transient influence is reversible, with all values approaching baseline within 30 minutes.