Firstly, this paper reviewed the effects of supplementary energy supply on nitrogen utilization, nitrogen kinetics in the rumen and urea metabolism in ruminants. Improving the utilization of dietary nitrogen by increasing microbial yield is possible through energy supplementation, but the type of supplement and level of intake required differ between diets. Secondly, the effects of sucrose supplementation on rumen fermentation and nitrogen kinetics were examined in sheep fed freshly cut lucerne, lucerne chaff and hay cubes. In this experiment, the method of isotope dilution of ¹⁴C-urea, ¹⁵N-urea, ³H-glucose and ¹⁵N-ammonium salt was used for the estimation of parameters of metabolism. Supplementation with sucrose decreased the urinary N excretion, resulting in an increase in N retention. Supplementation with sucrose resulted in a lower rumen ammonia and plasma urea concentration. The fermentation of sucrose in the rumen resulted in a decrease in rumen pH and in the ratio of acetate to propionate. Sucrose supplementation increased the proportion of urea transferred to the rumen, the non-ammonia N (NAN) concentration in the rumen and the NAN flow from the rumen to the lower digestive tract. Urinary allantoin excretion rate increased with sucrose supplementation. The plasma glucose concentration did not change but the plasma insulin concentration increased with sucrose supplementation. The influence of an energy-rich supplement on the utilization of N and N kinetics in the rumen and whole body of domestic ruminants is discussed.