Synaptic adhesion molecules of the cadherin superfamily have been proposed to control selective synapse formation. Classical cadherins, e.g. neural (N-) cadherin, mediate homophilic cell-cell adhesion and are found in the perisynaptic region. Because of the early embryonic lethality of N-cadherin knock-out mice, we established in vitro differentiation and immunoisolation of ES cell-derived neurons genetically null for N-cadherin. Glutamatergic synapses were studied in microisland cultures of these cells after 10–14 days in vitro electrophysiological methods. In paired recordings N-cadherin -/- neurons showed increased paired-pulse depression and enhanced depression during high frequency stimulation. To further investigate the possible importance of N-cadherin as target recognition molecule in competitive synapse formation, we studied chimeric co-cultures consisting of N-cadherin-expressing and N-cadherin-deficient neurons. Preliminary results indicate that glutamatergic synapses formed by a presynaptic N-cadherin expressing neuron and a postsynaptic N-cadherin deficient neuron show a similar defect in vesicle recruitment as described above. Intriguingly, glutamatergic synapses formed by a presnaptic N-cad -/- neuron and a postsynaptic N-cad +/+ neuron were almost undetectable, indicating a role in circuit formation.