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Timber transport by forest truck represents one of the largest energy consumers in the whole timber supply chain. Previous studies suggest that forest trucks spend as much as 14% of time driving on the primary forest roads, and that they consume a significant amount of energy in their total driving time. The fuel consumption is significantly impacted by the longitudinal slopes of the primary forest road network on which the timber is transported. This paper presents the results of research on fuel consumption by forest trucks while driving unloaded and loaded, depending on the longitudinal slope of forest roads. While driving an unloaded forest truck, increase in slope increases fuel consumption exponentially, and ranges from .7 L/km at an inclination of 3.5% to 1.6 L/km at an inclination of 9.5%. The fuel consumption of a loaded truck tends to increase exponentially with an increase of slope, and ranges from 1.21 L/km with 0% slope to 3 L/km with 8% slope. It can be concluded that a nominally loaded forest truck consumes on average 2.5 times more fuel on the same longitudinal slope in relation to the unloaded vehicle. The results show a strong link between fuel consumption and longitudinal slope. Carbon dioxide emissions are several times lower when the loaded forest truck is driven down the slope, when compared to being driven up the slope. [ABSTRACT FROM AUTHOR]