Here we report on pyramidal and reticulospinal excitation in forelimb motoneurones in the adult mouse using intracellular recordings in vivo (Alstermark and Ogawa, J Neurophysiol. 2004, 92:1958–62). The results have been obtained in BALB/C mice, which were anaesthetized with midazolam fentanyl/fluanison. In contrast to the rat, only weak and infrequent pyramidal excitation could be evoked with a minimal trisynaptic linkage. Disynaptic reticulospinal excitation could always be evoked as well as monosynaptic excitation from the medial longitudinal fasciculus. The results suggest that the reticulospinal pathway in the mouse is important in voluntary motor control of the forelimbs and that the role of the corticospinal tract might be different in mouse compared to rat. Our study provides an opening for studying the effect of genetic manipulation on specified descending systems in the mouse in vivo.