The relationship between adult speech and body movements of full-term healthy newborns was analyzed with a microcomputer (TM990/101M). The mother, pediatrician, and nurse were asked to talk with the subject, the infant, freely and also to read structured patterns such as "hi hi." There was a significant relationship between the infant's body movements and human voices (P less than 0.01). As a control, the infant was subjected to white noise, tapping sounds and to non-patterned sounds, where little relationship between body movements and non-human sounds was found. In 17 of 64 examined periods the infants reacted to the spoken voice with movement within 1.3 +/- 0.5 sec after the words. In 20/64, the mother spoke to the infant with a 1.4 +/- 0.4 sec latency after his movements. In 15/64, both the infant's movements and adult speech occurred at almost the same time with a 0.05 +/- 0.2 sec lag. This analytical method using a computer suggests not only the ability of a neonate to move his body synchronously with his mother's speech, but also that a mother also talks to her infant by reacting to his movements. This may be the basic process of language acquisition, and this method may have applications in the early diagnosis of some neurological diseases.