Using in vitro techniques, intracellular microelectrode recordings were made from the regions of origin of medial olivocochlear neurones in the rat auditory brainstem. Cells were characterised according to their action potential shape and their excitatory response to bath application of micromolar concentrations of the peptide neurotransmitter substance P and were filled with biocytin by ii!lection through the intracellular microelectrode for subsequent morphological reconstruction. Cells with a rapid component to the after-hyperpolarising phase of their action potentials (AHP2 cells) were most likely to show excitatory effects of substance P. Histological reconstruction showed that these cells were stellate with numerous large, slowly tapering dendrites exhibiting small, scattered spines. III examples in which the major axon was not cut near the cell body-the axons ascended dorsally out of the superiord olivary complex, in a manner that was consistent with the trajectory of axons of medial olivocochlear neurones. These features differed from other cells in the ventral nucleus of the trapezoid body that were unresponsive to substance P. In a further series of experiments, medial olivocochlear cells in the same region were retrogradely labeled by prior intracochlear injection with fast blue and recordings were made under direct visual observation using either microelectrode impalement or whole-cell patch methods. These data support the view that medial olivocochlear neurones are substance P-sensitive and exhibit a characteristic spike shape. These data strongly suggest that medial olivocochlear neurones possess receptors for substance P and may therefore receive excitatory input from a substance P-utilising neural pathway. (C) 1998 Elsevier Science B.V.