Presented is the final report of the ad hoc consultant group established to examine the advisability of NIMH (National Institute of Mental Health) making an explicit policy decision to reenter the field of mental retardation, to advise the two institutes (NIMH and NICHD--National Institute of Child Health and Human Development) regarding possible divisions of responsibility, and to indicate potential areas for cooperation and shared responsiblity. An introductory section reviews the scope of the problem and recommendations regarding research priorities, funding for research and training, and changes within the two institutes. Considered in Section II are the range of federal effort in mental retardation, background information on NIMH and NICHD, and involvement of university affiliated facilities and nonfederal organizations in the field. A third section focuses on the etiology of mental retardation, the most serious knowledge gaps that remain with respect to prevention, and other priority research areas. In a final section, current activity of and recommendations for NICHD with regard to mental retardation are discussed; ways in which NIMH and NICHD might complement one another's strengths are considered; and a priority list of mental retardation activities is given. (SBH)