Basic and acidic fibroblast growth factors (FGF) were implanted next to the proximal stump of the transected optic nerve of adult rats, in order to assess whether these molecules have neurotrophic activity in vivo. Of the 119,973 +/- 2484 (S.E.M.) retinal ganglion cells present in retinae of unoperated control rats, 11,375 +/- 2413 (S.E.M.) remained at 30 days after transection of the optic nerve in control operated rats. After implantation of gel foam soaked in basic FGF, the number of retinal ganglion cells surviving at 30 days after axotomy tripled (36,387 +/- 3270 (S.E.M.], after acidic FGF, it increased almost 4-fold (40,916 +/- 5405 (S.E.M.]. These results indicate that FGF has neurotrophic activity in the adult central nervous system, and that this molecule is able to rescue adult retinal ganglion cells from axotomy induced cell death. It remains to be shown whether FGF acts directly on retinal ganglion cells or indirectly via glial cells or other cells.