The effects of low, full (normal) and high birthweights on the broad range of neonatal behaviors measured by the Brazelton Neonatal Assessment Scale were investigated in a study which also attempted to replicate results of the authors' earlier study of the Brazelton Scale. Data from the original sample of 52 infants were included in the later study to make a total sample of 140 clinically normal newborns of over 37 weeks' gestational age. The infants' behaviors were scored on the 27-item Brazelton Scale. The principal component factor analysis of the second study yielded three main factors: attention-orientation, arousal and temperament. Results for the attention-orientation factor showed a significant main effect for birthweight, in that the mean score for the full-weight or normal group was higher than that for either the low or high groups. With low-weight neonates rated as a high-risk group for later developmental problems, it is hypothesized that a similar potential may exist for high-weight infants of normal gestational age. There were also some sex-birthweight interactions for the arousal and temperament factors, but these data are given a cautious interpretation because of certain characteristics of the sample. (BF)