The earliest biochemical change detected during synaptogenesis is a local elimination of muscle basal lamina proteins. To explore whether this provides signal(s) that regulate postsynaptic differentiation, we examined the effects of innervation on the distribution of β 1 -integrins, which were initially present in scattered aggregates complexed with basal lamina ligands. These β 1 -integrin aggregates disappear along paths of nerve contact as their basal lamina ligands are eliminated. New accumulations of these proteins then form during assembly of the postsynaptic apparatus. The new β 1 -integrin aggregates at developing synapses form partly via a redistribution of mobile molecules on muscle surface. We thus consider whether (a) the removal of integrins' basal lamina ligands alters their cytoplasmic ligand-interactions, causing the dissociation of integrin clusters, and (b) this receptor modulation helps to transduce local changes in pericellular protease activity into cytoplasmic signals that control postsynaptic differentiation.