Based on the demonstrated neurotrophic activity of VIP in vitro, a recently designed VIP antagonist was used to assess the role of this neuropeptide in the behavioral development of rats. Rats received daily subcutaneous injections from birth to day 14. Observations of developmental milestones/behaviors were made daily for 21 days. Of the measures of behavioral development tested, the time to surface right on day 4 and the day of onset for forelimb placing, hindlimb placing, forelimb grasping and air righting were significantly retarded by the antagonist. Cotreatment with VIP prevented the antagonist-induced delay. These results suggest that VIP activity is important in the development of select complex motor behaviors.