Abstractα‐Neurexins are synaptic cell‐surface molecules that are required for Ca2+‐triggered exocytosis. Mice lacking all three α‐neurexins show drastically reduced neurotransmitter release at excitatory and inhibitory synapses and die early postnatally. Although previous histological analysis of newborn α‐neurexin triple mutants revealed only a moderate reduction in the density of type II synapses in the brainstem, cell culture studies proposed that neurexins are prominently involved in synapse formation. To assess the contribution of α‐neurexins to the formation and structural properties of synapses in vivo, we performed a detailed morphological analysis of the brains from surviving adult double knockout mice lacking two of the three α‐neurexins. Despite their impaired neurotransmission, we did not observe any gross anatomical defects or changes in the distribution of synaptic proteins in adult mutants. Only mild structural alterations were found: a ∼20% reduction of neuropil area in many brain regions, resulting predominantly from shortened distal dendritic branches and fewer spines, as demonstrated by Golgi impregnation of pyramidal neurons. Quantitative electron microscopy revealed ultrastructurally normal type I and II terminals and a ∼30% decrease in the density of type II synapses in the neocortex. To exclude errors in pathfinding, we investigated axonal projections in the olfactory bulb of newborn knockouts and did not observe any changes. Therefore, α‐neurexins are not essential for the formation of the vast majority of synapses in vivo but rather regulate the function of these synapses. J. Comp. Neurol. 502:261–274, 2007. © 2007 Wiley‐Liss, Inc.