To evaluate the contributions of the pre- versus postsynaptic expression of NCAM in regulation of synaptic efficacy, we cultured dissociated hippocampal cells from NCAM-deficient and wild-type mice in homo- and heterogenotypic combinations. Double recordings from synaptically coupled neurons maintained in heterogenotypic cocultures showed that synaptic strength of excitatory but not inhibitory synapses depended on expression of NCAM post- but not presynaptically. This correlated with higher levels of potentiation and synaptic coverage of NCAM-expressing neurons compared to NCAM-deficient neurons in heterogenotypic cocultures. Synaptic density was the same in homogenotypic cultures of NCAM-deficient and wild-type neurons as well as in heterogenotypic cocultures in which glutamate receptors were blocked. These observations indicate that the relative levels of postsynaptic NCAM expression control synaptic strength in an activity-dependent manner by regulating the number of synapses.