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The three closely related sterols, cholesterol, 7-dehydrocholesterol and lathosterol, occur in animal tissues in different proportions. From their structures they would appear to be interconvertible by simple dehydrogenation or reduction steps as shown in Fig. 1, but unequivocal evidence for such a mechanism has not yet been obtained. It has generally been assumed that 7-dehydrocholesterol (provitamin D) is formed in vivo by dehydrogenation of cholesterol, and observations on the feeding of cholesterol to guinea pigs supported this concept (Glover, Glover & Morton, 1952). The work was done, however, before the discovery of lathosterol, which is difficult to separate from its isomer cholesterol. It has been shown that lathosterol can be converted into cholesterol in vivo (Cook, Kliman & Fieser, 1954) and in vitro (Frantz, Davidson, Dulit & Mobberley, 1959; Mercer & Glover, 1959), but the intermediates have not yet been established. The formation of cholesterol from zymosterol (Johnston & Bloch, 1957) and lathosterol (Frantz et al. 1959) requires aerobic conditions, which has tentatively been interpreted as perhaps involving hydroxylation followed by a dehydration step. This processwould not easily be reversible. The rapid and extensive reduction of 7-dehydrocholesterol to lathosterol that takes place in the guinea pig and in the rat (Glover & Green, 1957; Glover & Stainer, 1959)

Sterol metabolism. 6. The interconversion of cholesterol, 7-dehydrocholesterol and lathosterol in the rat