Localized injections of the mu antagonist CTOP into intracisternal (i.c.) or intracerebroventricular (i.c.v.) sites altered the behavior of 1-day-old rat pups during continuous exposure to an artificial nipple. Blockade of mu opioid receptors by i.c. injection decreased oral responsiveness to the nipple, while blockade of receptors by i.c.v. injection of CTOP increased oral responsiveness. The injection of CTOP into the i.c. site produced a transient reduction in body weight gain in pups suckling from their mother, while injection into the i.c.v. site had no effect. When cesarean-delivered pups were tested prior to suckling, injection of CTOP into the i.c. site increased latency of the first nipple attachment and decreased total time attached to a surrogate nipple providing milk. Injection of CTOP into the i.c.v. site decreased latency to the first nipple attachment. The results indicate that there is a caudal population of opioid receptors that is involved in the initiation of suckling behavior and a rostral population that plays a role in decreasing responsiveness at the nipple.