The refractive index variations along the optic axes of nine foetal bovine lenses were measured using an optic fibre reflectometer. The device measures refractive index directly, using the principle of Fresnel reflectance. These results show that lenses with a wet weight >0.7 g (corresponding to 4.5 months gestational age), have a refractive index profile which approximates a parabolic shape, akin to that found in post-natal and adult bovine lenses. In lenses of wet weight below 0.7 g, the refractive index distributions show irregularities. These findings suggest that the refractive index profile in the bovine lens starts to form into a parabolic distribution, as seen in adult lenses, about half-way through gestation. The trigger for this formation is not known. (C) 2003 Elsevier Ltd. All rights reserved.