Fragile-X, the main cause of inherited human mental retardation is associated with the absence of a recently identified fragile-X mental retardation protein (FMRP). Mice in which this protein is lacking due to a knockout (KO) mutation are reported to express altered dendritic spines on their cortical neurons compared with wild type (WT) controls. We have used tissue-cultured neurons to examine differences in morphology and synaptic connectivity between WT and FMRP-deficient mice. Hippocampal neurons taken from KO mice and grown in culture for 3 weeks have shorter dendrites and fewer dendritic spines than their WT counterparts. Also, KO cells tend to express fewer functional synaptic connections, which develop more slowly and produce smaller excitatory synaptic currents than WT controls. These observations may have important implications for the understanding of mental retardation associated with the absence of FMRP.