Previously we reported that, in vitro, lens cells proliferate, migrate or differentiate in response to low, medium and high concentrations of FGF respectively. To examine further the role of FGF in lens development we used immunohistochemistry to study the distribution of aFGF and bFGF in the eye of the 20 day rat foetus. Strong aFGF-like reactivity was localised in a band of cells near the lens equator which included the germinative zone where most cell proliferation occurs and the transitional zone where epithelial cells differentiate into fibres. The closely apposed inner epithelial layer of the ciliary and iridial retina also reacted strongly. Reactivity for aFGF was also found in the epidermis and in the corneal and conjunctival epithelia. In the neural retina, reactivity was found in the nerve fibre layer and in isolated cells of the inner plexiform layer. bFGF-like reactivity was found in the retinal ganglion cell layer, extra-ocular muscles and associated with endothelial cells of the hyaloid, lenticular and choroid vasculatures. Pre-digestion of sections with hyaluronidase caused loss of cell-associated reactivity but revealed strong bFGF-like reactivity in ocular basement membranes, in particular, the lens capsule. The sensitivity of this capsular bFGF localisation to heparinase indicates that bFGF in the extracellular matrix is complexed with heparan sulphate proteoglycans. The results of this study are consistent with the hypothesis that FGF plays an important role in lens development via both autocrine and paracrine mechanisms.