Although great importance has been attached to the study of the toxicity of silica, mainly the involvement of silica in silicosis, there had been little work concerned with the effect of silicon in normal metabolism and there was no evidence that there was any need for silicon in higher animals until silicon was reported to be an essential trace element in 1972 as a result of earlier studies in this laboratory. Silicon was shown to be required for growth and skeletal development in the chick. The increase in growth was marked suggesting that silicon must play an important role in the body's metabolic processes. Our later work shows that silicon must play an important role in normal functioning of connective tissues as we have shown silicon to be involved in several aspects of connective tissue metabolism such as (1) in connective tissue formation (2) as a structural component (3) in aging of certain connective tissues (4) in bone calcification processes and recently (5) at the cellular and subcellular level. The objectives of this research are then to establish the physiological role of silicon, that is to establish the function, site and mode of action of this trace element. We propose therefore to make an intensive investigation into the role of silicon in connective tissue metabolism with emphasis on aorta and bone. In vivo nutritional studies using chicks and rats and in vitro studies including isolation and quantitative analyses of connective tissue components, electron microprobe and tissue culture will be carried out to attain these objectives.