Regulated traffic of functionally differentiated transmitter receptor subtypes appears to be a general phenomenon during development and plasticity of synapses. To address in vivo the mechanistic principles of this process, we use glutamatergic synapses at neuromuscular junctions (NMJs) of Drosophila which resemble glutamatergic synapses of mammals in size and molecular composition. At the NMJ, high conductance (GluRIIA) and low conductance glutamate receptor (GluRIIB) complexes are found co-expressed within individual postsynaptic densities (PSDs). We imaged the dynamics of fluorescence labelled glutamate receptors during PSD assembly. At nascent PSDs, GluRIIA/GluRIIB composition was heterogeneous but converged towards an equalized composition during further growth. FRAP analysis showed that GluRIIB complexes incorporated uniformly but reversibly at all PSDs. In contrast, GluRIIA incorporated strongly and irreversibly at nascent PSDs but hardly at mature PSDs. With a combination of stochastic and deterministic simulations to develop quantitative markovian models of receptor incorporation during PSD maturation we find, that the postsynaptic conductance established at nascent PSDs controls further incorporation of highly conductive GluRs. This feedback might allow to couple presynaptic release of glutamate with PSD maturation and synapse formation at developing synapses.