Cataracts, inherited in an autosoraal dominant manner in New Zealand Romney sheep were first detected at 1 - 2 months of age as focal anterior and posterior cortical opacities. Within a short time these coalesced to form spoke-like patterns which gradually became more diffuse. Total lens opacity developed by 10 - 11 months of age. Congenital mature cataracts occurred in two lambs believed to be homozygous for the mutant gene and in these animals lens resorption later occurred.Histology showed that new lens fibres formed normally at the equator but then underwent progressive degenerative swelling and lysis. Cytoplasmic vacuolation of anterior epithelial cells was a notable feature and occurred apparently in response to adjacent cortical degeneration. Ultrastructurally these vacuoles were interpreted as dilations of endoplasmic reticulum but there were also a number of smaller vacuoles not lined by membrane. Anterior epithelial cells also underwent metaplasia, hyperplasia, and migration beneath the posterior capsule. Analyses of lenses reflecting different stages of cataracto-genesis showed a progressive increase in water, sodium and calcium ions with cataract development and a concomitant decrease in potassium and magnesium ions.