A protein (FMRP) which is lacking in patients with Fragile X Mental Retardation is also not produced in fmr-1 KO mice, a model for the syndrome. In previous work, we showed that fmr-1 KO mice do not initiate increased synaptic translation in response to glutamate neurotransmitter stimulation (Spangler et al., SfN 1998). Moreover, these KO mice exhibit lower numbers of polyribosomes in postsynaptic spines. Thus the absence of FMRP seems to be associated with a translational defect; it seems likely that a specific subset of proteins might be affected . We have used Western blots of proteins from synaptoneurosomes from 3 brain areas (cortex, cerebellum, hippocampus) of mice aged 6 days to 4 weeks, to compare levels of glutamate receptors and other proteins during development. Each of the synaptic proteins tested (AMPA receptors GluR1, GluR1; mGluR1; GABA A) is present, on average, at slightly lower levels in KO cortex than in WT cortex, in young mice. There is a particularly striking cortical deficit in mGluR...