It seems appropriate to start a discussion of visual mechanisms with some comments on rhodopsin, the molecule which transduces light energy into the nervous impulse leading to vision. In particular, I shall discuss those aspects of rhodopsin which are concerned with its lipoprotein nature. Lipoproteins as a class of complex molecules were first isolated in a pure form thirty years ago,11 but we still know almost nothing about the mechanisms of association between the lipide and the protein portions of such molecules. The most common picture is of a molecule held together by electrostatic interaction between oppositely charged groups of the protein and phospholipides, which in turn, bind neutral lipides by secondary valence forces. Despite this lack of precision in describing the mechanism of combination of lipides with proteins, a number of lipoproteins have been isolated with constant composition. The best known of these are present in human plasma,