Synchronization of the ruminal degradation of carbohydrates and CP is projected to increase ruminal microbial protein synthesis and improve N use efficiency. Attempts to synchronize the fermentation of dietary carbohydrates and CP have been met with mixed results, suggesting that ruminal nutrient synchrony is not important or that physiological mechanisms work in concert to synchronize ruminal carbohydrate and N availability. Nitrogen recycling to the rumen is controlled primarily by the concentration of urea in the blood, ammonia in the gut, and the availability of fermentable energy in the gut. We hypothesized that N utilization could be improved by synchronizing the supply of nutrients in one segment of the gut with those in another segment (i.e., synchronize a ruminal N deficiency with a lower gut N excess, etc.) via oscillating the dietary CP between deficient and adequate concentrations. With corn-based diets and oil-seed-based natural protein supplements, N retention has been greater in lambs or steers fed oscillating CP concentrations (at 48-h intervals) than in animals fed a constant CP percentage. Effects of oscillating CP on cattle performance have been variable and may depend upon the fermentability of the carbohydrate source (e.g., forage vs. grain, grain processing). Studies with sheep noted that net portal uptake of urea was greater in lambs fed oscillating CP than in lambs fed constant CP concentrations. Nutrient intakes also need to be synchronized with the animals' requirements. One method to adjust nutrient intake with requirements is via phase-feeding. Results of studies with dry-rolled corn-based diets indicate that dietary CP concentrations can be decreased late in the feeding period with no adverse effects on animal performance; however, results of studies using steam-flaked corn-based diets are less consistent, possibly due to differences in the aggressiveness of the implant program used. In conclusion, ruminal nutrient synchrony is theoretically a sound principle; however, it seems that physiological mechanisms such as N recycling may mitigate effects of asynchrony. Methodologies that increase N recycling or increase the utilization of recycled N may benefit animal performance and the environment.