gesoittino Acids of the exrerinent. In some mote olic srocesses, certain compounds may de essential intermediates and yet never exist at any cic time in any but very minute quantities, excessive amounts of such intermediates bein; metabolizec by other paths. FProolems posed by a situation of this kind can be met by using known precursors cf greater stability with respect to transformations of no interest in the particular experiment. These foregoing points will bz illustrated by examples from the literature (work of Schosnheimer, Rittenberg, Du Vignead, Rorsook, and others). The remainder of ths hour will be devoted to a presentation and discussion of other published werk end cf work being done by groups at the -alifornia Institute of Technology, illustrating the extreme ubility of isotcpes in the study of the metabolism of arins acids. David R. Howton.