The effect of energy source in diets on duodenal flow of bacteria N and fatty acids was measured in an experiment on four sheep in a 4x4 Latin square design. The basal diet for control animals consisting of 40% concentrate and 60% meadow hay was supplemented for experimental animals with 6% of rape seed oil, saccharose, or starch as an additional energy source. The energy supplement did not change rumen pH (P>0.05), but decreased (P<0.05) rumen ammonia-N from 41.6 mmol in the control group to 15.6,17.5 and 23.3 mmol in groups receiving rape seed oil, saccharose, or starch, respectively. A similar declining tendency was found for total volatile fatty acids and the majority of individual ones. Duodenal flow of bacteria N calculated from 2,6-diaminopimelic acid content was not significantly affected by the diet. The ratio of 2,6-diaminopimelic acid to total nitrogen content in digesta was lower in control than experimental groups, indicating a higher proportion of bacterial protein in the digesta of animals receiving additional energy in diets. The proportion of unsaturated as well as saturated fatty acids in the digesta of animals fed diets supplemented with starch was lower (P<0.05) than in the remaining groups. The duodenal digesta of animals fed the diet supplemented with rape seed oil contained significantly higher (P<0.05) polyunsaturated n-3, monounsaturated and saturated fatty acid proportions compared with the remaining groups. These results demonstrate that feeding sheep with diets supplemented with energy decreases ammonia and volatile fatty acid levels in the rumen fluid whereas only addition of rape seed oil increases the proportions of polyunsaturated n-3, monounsaturated and saturated fatty acid in the fat of duodenal digesta.