We consider here the behaviour of wood ﬁbre suspension with ﬁbre concentration above that of sedimentation in a pressure driven ﬂow in a straight pipe with smooth walls. The ﬂow behaviour can be roughly divided in two main regimes: the plug ﬂow regime that occurs at low ﬂow rates and the drag reduction regime that occurs at high ﬂow rates. We utilized new experimental methods in order to gain more detailed understanding on the ﬂow behaviour of wood ﬁbre suspensions, and especially on the relevant physical phenomena inducing such behaviour. In addition to carrying out conventional loss experiment, the velocity proﬁles across the pipe were measured using pulsed ultrasound velocimetry (PUDV) techniques, and the thickness of the lubrication layer in fully developed ﬂow was measured using a laseroptical device. Based on our direct measurements, we were able to indentify ﬁve different ﬂow regimes in suspension ﬂows. In addition, we reﬁned the qualitative picture of these ﬂows in relation to the forming of ﬁbre plug and to the physical phenomena taking place in transition from one ﬂow regime to another one.