From the experimental results presented, some of the limiting factors as well as characteristic features regarding the mechanism whereby carbohydrate spares nitrogen in the normal nutritive state may now be pointed out: The single administration of extra carbohydrate in the normal nutritive state spares nitrogen only when it is ingested at a definite time in relation to the ingestion of protein. When the extra carbohydrate is administered at intervals far removed from the time of protein feeding, no nitrogen retention occurs even though the amount of extra carbohydrate represents 200 calories or approximately one-third of the daily caloric intake. The interval during which the extra carbohydrate is able to exert a nitrogen-sparing action is limited to 4 hours before and 4 hours after the ingestion of the daily meal. In every case it is the nitrogen to which the protein of the meal contributes that is affected by the extra carbohydrate. This observation thus establishes a new factor, namely, one of time, in the protein-sparing action of carbohydrate in the normal nutritive state. Since, as judged by the nitrogen excretion, the metabolism of protein proceeds most rapidly in the early hours after its ingestion (Lusk, ′28 b), this time factor suggests that carbohydrate effectively spares protein only if it is available to the animal at a time when an increase protein metabolism is in progress. Not only the occurrence but also the extent of the nitrogen retention produced by extra carbohydrate administered during the normal nutritive state are determined by the time interval between the ingestion of protein and that of carbohydrate. The most pronounced sparing of nitrogen occurs when the extra carbohydrate is ingested at the same time as the meal or within 1 hour after it. As the interval between the ingestion of these two foodstuffs is lengthened, the nitrogen spared by equal amounts of carbohydrate falls off until finally, when the interval between the two is greater than 4 hours, no sparing whatever occurs. Despite the fact that the 24-hour nitrogen output may show a well-defined sparing of nitrogen in response to a single administration of extra carbohydrate, a decreased nitrogen excretion does not characterize the whole of the 24-hour period during which the carbohydrate is ingested. The interval in which nitrogen retention occurs after a single administration of 50 gm. of extra carbohydrate is very short. In the present study it was observed during the first 12 hours following the carbohydrate feeding. The nitrogen eliminated in the next 12 hours had already shown a rise above the previously established equilibrium level for this period. The nitrogen spared under the influence of a single feeding of extra carbohydrate is therefore not permanent, its elimination beginning several hours after its storage is effected. Repeated daily additions of extra carbohydrate at the same time that protein is ingested result in nitrogen retention, but this continues only as long as the daily administrations of extra carbohydrate are maintained. The retention of this nitrogen requires the continued presence of the extra carbohydrate, for, although nitrogen continued to be deposited for 7 days under the influence of repeated administrations of extra carbohydrate, the cessation of this extra carbohydrate results in an immediate outpouring of increased amounts of nitrogen, a process that continues for several days after the feeding of extra carbohydrate is discontinued. Here again, despite the fact that the administration of extra carbohydrate was repeated daily for 7 days, actual storage of nitrogen occurred only during the first few hours following the carbohydrate intake, the nitrogen excretion during the later intervals of the 24-hour period being relatively unaffected or even increased. Similarly, when an increased outpouring of nitrogen results from the cessation of daily carbohydrate treatments, this takes place in the interval corresponding to that in which the nitrogen storage is observed (i.e., the first 12 hours), the nitrogen excretion of the later periods remaining relatively unaffected. The fact that an increased nitrogen output in the early hours after protein ingestion was observed for several days after an interruption of extra carbohydrate feeding indicates that the immediate fate of ingested protein is determined in part by the animal’s previous carbohydrate intake. This observation is in harmony with the view that nitrogen metabolism in the animal is in a ‘continuing’ state, as recently pointed out by Borsook and Keighley (’35).