To investigate cross-sectionally and longitudinally the development of children's early verbal behavior, 10 normal first-born Japanese infants from middle-class families were observed monthly and examined from when they were 6 to 24 months of age. Audio- and video-taped language data were obtained in a laboratory playroom setting during 15-minute sessions of free play with the mother present. Neonatal and postnatal neurological examination records were obtained to establish neonatal activity levels. Infants' scores at 6, 12, 18, and 24 months on the MCC Baby Test (a modification of an instrument by Cattell) were obtained to provide developmental data on search behavior. Of particular interest were (1) the relationship between the time of emergence of the first meaningful word and the emergence of two-word utterances, (2) the relationship between cognitive and language variables, (3) the influence of neonatal activity level upon cognitive and language development, and (4) the relationship of maternal verbal functions at 12 months with cognitive and language development up to 24 months. A correlational analysis was made of 39 variables of language and cognitive development, neonatal activity level, and maternal verbal functions at 12 months. Results are discussed in relationship to previous studies and the need for further research is indicated. (Author/RH)