Infant's developmental changes in cerebral lateralization in response to speech stimuli were assessed using a multi-channel near infrared spectroscopic measurement system. Analysis-synthesized Japanese word stimuli were used as phoneme (/itta/ and /itte/) and pitch (/itta/ and /itta?/) contrasts. The baseline blocks contained only /itta/, whereas /itta/ and its phoneme or pitch counterpart were presented randomly with equal probability during the contrast blocks in each session. To analyze cerebral lateralization, we calculated laterality indices using maximal total-Hb changes during the co...