A pre-embedding double immunostaining technique was used to determine the role of βb-endorphin in synapse, particularly in neurons with a postsynaptic membrane containing μ-1 orβ-1 opioid receptors. A small number of β-endorphin immunoreactive axon terminals in the dorsal raphe nucleus was found to make direct synapses on μ-1 or δ-1 opioid receptor-immunoreactive dendrites, some of which showed immunostaining of their postsynaptic membranes, although with low frequencies. These results suggest that β-endorphin can play a direct role through the μ-1 or δ-1 opioid receptors at synapses, but the main route would be through other opioid receptor at the synapse or even not through the synapse.