Infant and adult rats were injected chronically with either naloxone or saline for 21 consecutive days. At various intervals after cessation of the pretreatment with naloxone, animals were sacrificed and assessed for specific binding of [3H]naloxone in different regions of the CNS. Infants displayed an increase in opiate binding in the spinal cord, hypothalamus, striatum and cortex one day after cessation of the pretreatment with naloxone, but the increase in opiate binding was dissipated within one week after cessation of the pretreatment. The increase in opiate binding in infants was accompanied by an increase in the antinociceptive efficacy of morphine. In contrast to infants, adults failed to display any alteration in opiate binding following the chronic pretreatment with naloxone. Infants may be especially susceptible to naloxone-induced receptor supersensitivity because infants excrete naloxone more slowly than adults, and thus their opiate receptors may be blocked for a longer duration following an injection of naloxone.