1. Intracellular recordings were made from 123 tonically active medial vestibular nucleus (MVN) neurones in a horizontal slice preparation of the dorsal brainstem of the rat. On the basis of their averaged action potential shapes, the cells were classified as either type A, having a single deep after-hyperpolarization (AHP; 40/123 cells, 33%), or type B, having an early fast AHP and a delayed slow AHP (83/123 cells, 67%). The two cell types were distributed throughout the rostrocaudal extent of the MVN. 2. In type A cells TEA reduced the single deep AHP and decreased the rate of spike repolarization. Depolarizing current pulses from a hyperpolarized membrane potential elicited spikes with short plateau potentials in TEA. These persisted in Ca(2+)-free medium but were abolished along with the spontaneous activity in TTX. Ca(2+)-free medium did not affect the initial rate of repolarization but reduced the deep AHP. Apamin and carbachol had little effect. 4-Aminopyridine (4-AP) slowed spike repolarization and the AHP amplitude by a small amount. Thus, in type A cells spike repolarization and AHP appear to be mediated largely by a TEA-sensitive potassium current (presumably IK) and an apamin-insensitive Ca(2+)-activated potassium current (presumably IC). 3. The early fast AHP in type B cells was readily abolished in TEA. In seven of ten type B cells tested, the spontaneous spikes developed plateau potentials of 100-120 ms duration in 10 mM TEA, which then became 7-9 s long in Ca(2+)-free medium. In the remaining three cells, the spontaneous plateaux were 1.75-2 s long in TEA, and were reduced to 30-100 ms in Ca(2+)-free medium. TTX abolished the spontaneous spikes and plateaux. The delayed AHP was abolished by apamin, which induced irregular firing. 4-AP slowed spike repolarization and abolished the fast AHP, but did not induce plateaux. Thus, in type B cells spike repolarization involves a TEA-sensitive current (presumably IK) as well as IC and the 4-AP-sensitive potassium current IA, while the apamin-sensitive potassium current IAHP is responsible for the delayed AHP. 4. The tonic activity in type B cells appears to be regulated mainly by interactions between a persistent Na+ current, which in most cells is large enough to generate plateaux when repolarization is impeded in TEA, and the hyperpolarization mediated by IAHP. About 30% of type B cells have an additional inward Ca2+ current.(ABSTRACT TRUNCATED AT 400 WORDS)