Large numbers of human infants are exposed in utero to illicit opiate drugs such as heroin and to prescribed opiates such as methadone. It is therefore important to understand the effects of acute and chronic exposure to drugs such as these on the immature organism. A necessary step in understanding the long-term effects of early drug exposure is to describe fully how the drug acts on neurobehavioral systems in the neonate. Because infants typically experience withdrawal from opiates, either in utero or shortly after parturition, a fuller understanding of the consequences of opiate withdrawal in the immature animal is an important but largely unexplored area of research. Although there has been some question as to whether infants undergo withdrawal, more recent experiments demonstrate that the rat neonate experiences clear behavioral changes during opiate abstinence, including the induction of "dysphoric" states on cessation of drug exposure. It is proposed that the infant organism undergoes withdrawal but that the signs and symptoms experienced differ from those suffered by the adult.