The ontogeny of spontaneous motor and sensorimotor behaviors of preweaning rats, as well as ambulation, emotionality, and nociception at weaning (day 21), were studied in rats given chronic administration of 1 or 50 mg/kg naltrexone from birth to day 21. The age at which a specific spontaneous motor behavior or performance initially appeared and the age at which 100% of the animals demonstrated a particular behavior were accelerated in animals given 50 mg/kg naltrexone, but delayed in rats injected with 1 mg/kg naltrexone. In general, ambulation, emotionality, and nociceptive responses were not affected by naltrexone treatment, although the frequency of face-washing in both naltrexone groups and activity cage performance in the 50 mg/kg naltrexone group deviated from control levels. Observations of head-shake and wet-dog shake behaviors in naltrexone-treated animals at 2 hr and 10 hr post-drug injection were similar to controls with the exception of an abnormal increase in the 1 mg/kg naltrexone group at 10 hr. Although these results may imply that endogenous opioid systems play a role in regulating neurobehavioral development, further study is needed to distinguish whether these changes are a consequence of the somatic and morphological alterations known to occur with naltrexone administration or if the timetable of behavioral ontogeny is governed by endorphin-opiate receptor interaction.