Exposure to opiates such as morphine can lead to psychological and physical dependence in both adult and infant humans. Infant rats experience opiate withdrawal behaviors that are qualitatively different from the withdrawal behaviors displayed by adult rats. In the adult, withdrawal is largely mediated by the mu-opioid receptor. We sought to understand more about what role each opioid receptor (mu, kappa, and delta) plays in the display of the physical withdrawal in the infant rat. Beginning on postnatal day 1, infant rats were injected with morphine sulfate twice a day for 6.5 days. On the afternoon of the seventh day the infant rats were given an i.c. injection of a vehicle, the mu-opioid receptor antagonist CTOP, the kappa-opioid receptor antagonist nor-BNI, or the delta-opioid receptor antagonist naltrindole. CTOP precipitated withdrawal behaviors in the 7-day-old rat in a dose-dependent manner. Neither nor-BNI nor naltrindole induced any significant changes in the frequency of the withdrawal behaviors. These data suggest that in the infant rat control of certain behavioral withdrawal signs is modulated primarily by the mu-opioid receptor, as is the case in the adult rat.