The abducens nucleus in carpet sharks is not a discrete delimited nucleus, as the dendrites of the motoneurons extend into the reticular formation and the medial longitudinal fasciculus. lnjections of horseradish peroxidase (HRP) designed to trace the inputs to these neurons are therefore difficult to confine to this system alone. Despite this problem a consistent finding from injection of HRP in the area of the abducens nucleus is the retrograde labelling of a column of cells in the contralateral octaval nuclei. The column of cells is predominantly in the ventral portion of the descending octaval nucleus, but does straddle the entrance of nerve VIII, extending into the caudal part of the ascending octaval nucleus. Labelled cells correspond in location and morphology to those cells receiving input from horizontal canal afferent fibers, confirming the trineuronal nature of the horizontal vestibulo-ocular reflex arc in elasmobranch fishes.