The neonatal period is being recognized as an important period for the development of patterns of interaction between mother and infant, and infant state has been shown to have a significant impact on mother--infant interactions. A major dimension of infant state, with implications for this interaction and for the development of later behaviour disorders, is the infant's irritability. Research with Navajo, Malay, Chinese and Tamil mothers and infants showed that normal variation in maternal blood pressure during pregnancy was related to newborn irritability as assessed with the Brazelton Scale. This relationship is discussed in terms of possible underlying mechanisms.