The death of facial motoneurons after axotomy provides a useful tool for studying neurotrophic factors which could prevent motoneuron loss in vivo. The right facial nerve trunk before the postauricular nerve branching of newborn rats was transected at its extracranial exit, and topically treated, at the axotomy site, with either a vehicle solution containing agarose or sucralfate, or acidic fibroblast growth factor (aFGF). Acidic FGF treatment increased the survival of the facial motoneurons from 18% to 70%. These results suggest that aFGF is a neurotrophic factor for motoneurons in vivo and that this growth factor may provide a new basis for the development of treatments to prevent the loss of damaged motoneurons.