Protein distribution patterns were determined in concentric layers removed from 24 bovine lenses ranging in age from about 6 months before birth to 180 months post-natal. It was possible to distinguish alterations in protein synthesis patterns during development and changes due to ageing, i.e., prolonged existence of the proteins. It was found thatα-crystallin represents a constant 50% of the proteins synthesized by the fibre cells throughout life. However, the protein becomes progressively less soluble with increasing age.β-crystallin synthesis increases from 30% of the total proteins during prenatal development to around 40% in post-natal fibre cells. This increase is due to increased production of theβI-crystallin. In old tissues,βH-crystallin is converted to a high molecular weight form (HMWβ)γ-crystallins account for 22% of the proteins synthesized in the earliest prenatal fibre cells. This level decreases rapidly through prenatal development until they represent about 4% of the total at birth.βS-crystallin synthesis commences around this time and in the post-natal fibre cells is essentially the only low molecular weight protein. The possible significance of some of these changes is discussed with regard to the functional requirements of the lens.