This report highlights some of the projects supported by the Mental Retardation and Developmental Disabilities (MRDD) Branch of the Center for Research for Mothers and Children at the National Institute of Child Health and Human Development since its last report in January 1997. The MRDD Branch provides support for research, research training, conferences, and workshops on the biomedical, behavioral, and bio-behavioral aspects of mental retardation and other forms of developmental disabilities. It employs the following mechanisms to support research in the biological, behavioral, and social processes implicated in the understanding, prevention, and treatment of mental retardation and developmental disabilities: individual research grants, program projects, core support for the Mental Retardation Research Centers (MRRCs), contracts, training grants, and conference grants. The report describes the activities of the individual MRRCs and research activities addressing chromosome abnormalities, prenatal malnutrition, maternal phenylketonuria, family functioning, autism, and prenatal diagnosis. The report also includes trends of support for the MRDD branch for the past five fiscal years. The level of support of $105.1 million provided by the Branch in fiscal year 2000 represents an increase of 163 percent, compared to $64.4 million in fiscal year 1996. Appendices list sponsored conferences and workshops. (CR)