The timecourse of the appearance and maturation of intrinsic tonic activity in medial vestibular nucleus (MVN) neurones was examined using extracellular single unit recording techniques in slices of the dorsal brainstem prepared from balb/c mice at specific stages during the first postnatal month. In slices from animals at postnatal day 5 (P5), the intrinsic spontaneous discharge rate was low (< 5 impulses/s on average). Over the period P10 to P30 this gradually increased to levels comparable to those of adult MVN cells in vitro. There was a rostro-caudal gradient in the time-course of development of tonic activity, such that cells located rostrally within the MVN developed higher frequencies of tonic discharge earlier than caudally located cells. The opening of the eyes around P14 was associated with a significant increase in the mean discharge rate of caudally located, but not rostrally located, MVN cells.