The tetracyclic compound epinastine (3-amino-9, 13b-dihydro-1H-dibenz(c,f)imidazo(1,5a)azepine hydrochloride) that was recently introduced as a vertebrate histamine H1 receptor antagonist has also high affinity for insect neuronal octopamine receptors. This holds true for the neuronal octopamine receptor from the locust (Ki = 2 Nm) as well as from the honey bee nervous system (Ki = 1.1 Nm). In addition to its high affinity, it has a high degree of specificity. Its affinity for other insect receptors for biogenic amines, such as 5-hydroxytryptamine, dopamine, histamine, and tyramine, is at least four orders of magnitude lower. Therefore, epinastine could serve as a highly specific antagonist of octopamine receptors that enables physiological dissection of octopaminergic neurotransmission within the nervous system of insects. To demonstrate these abilities, epinastine was used to inhibit the visually evoked activity of an identified interneuron in the visual pathway which is known to be modulated by octopamine.