Experiments were performed with anaesthetized (urethan and chloralose) spontaneously breathing rabbits. Horseradish peroxidase (HRP) was injected (1 mg) with a Hamilton syringe into the end of the sectioned mylohyoid nerve. The HRP-labelled neurones could be localized in the motor nucleus of the trigeminal nerve (N.V.mt.). It was found that the labelled neurones are present in the caudal and intermediate part of the nucleus. However, their largest agglomerations were noted in the <caudal part>, where they occupied the entire frontal section of the nucleus. It is assumed that the N.V.mt. directing their axons to the muscles of the upper <respiratory airways, simultaneously send collaterals to the central respiratory neurones participating in the regulation of the respiratory rhythm. According to this hypothesis the neurones of the M.V.mt. may ensure synchronisation between the upper and lower respiratory system.