PLANT protein in the fodder of ruminants does not arrive unaltered at the abomasum where peptic digestion takes place; there can be no doubt that at least a part of it is degraded, and later resynthesized to microbial protein by the enormous numbers of bacteria and other micro-organisms in the rumen. From time to time attempts have been made, usually on a basis of insufficient data, to calculate the extent of this conversion, and the estimates have varied from almost negligible proportions to 100 per cent conversion1. Two more recent attempts2 have involved the feeding of special proteins—zein and casein. No data are as yet available, however, which allow an accurate estimate to be made of the extent to which this conversion takes place in animals fed on natural fodders. In the course of certain studies of the functions of the omasum in the ruminant stomach (to be published elsewhere) we have had occasion to examine the distribution of nitrogen in the rumen contents of slaughtered sheep which had previously been fed on wheaten hay, and some of the results, together with analyses of the fodders, provide interesting information on the quantitative aspect of the conversion of plant to microbial nitrogenous compounds.