Summary Twenty pregnant heifers and cows were paired according to breed and to number and stage of gestation. For 7 weeks (average) before parturition, one group received a high-protein ration and the other a low-protein ration. The rations did not effect a significant difference in the levels of total protein, of casein, and of albumin-globulin fractions of colostrum and early milk from the foregoing groups. Non-protein nitrogen levels were higher in colostrum and early milk from cows of the high-protein group, but the differences were significant only in samples after the first three collections postpartum. The decrease in concentrations of the protein fractions of mammary secretions during the transition period tended to follow a logarithmic curve for the first four to six milkings, after which the rate of decline was less rapid. Changes in non-protein nitrogen seemed to continue at approximately the same logarithmic rate in samples representing the second through the sixteenth milkings. Rates of change of the nitrogenous constituents were similar in colostrum and in milk from cows receiving either the high- or the low-protein rations. Analysis of intra-group data indicated that colostrum from first-lactation cows receiving the high-protein ration contained higher levels of albumin-globulin nitrogen than did second-lactation cows receiving the same ration. Only small differences were observed after the first four milkings. Similar differences between heifers and cows fed the low-protein ration were not evident; the values for both of these lactation groups were between those of the heifers and cows receiving the high-protein ration. Deviations from the mean of values for nitrogen fractions for individual cows within the various groups were considerably greater during the early colostral period than later, as the composition of milk approached normal. Total protein and albumin-globulin contents of early colostrum were related to degree of mammary edema only to the extent that the averages of each of these protein fractions were higher from the ten cows judged to have the more severe edema than from the ten cows with the less severe edema. The rate of decline of total protein and of albumin-globulin contents of mammary secretions during the transition period was not related to the degree of mammary edema.